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OPTICS LETTERS / Vol. 39, No. 19 / October 1, 2014
Injection-locked operation of an optically pumped semiconductor laser Yi-Ying Lai,1,2,* Yevgeniy Merzlyak,1,2 J. M. Yarborough,1,2 Kevin Winn,1,2 and Yushi Kaneda1,2 1
College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA 2 ERATO, Katori Innovative Space-Time Project, Tokyo 113-8656, Japan *Corresponding author: [email protected] Received July 17, 2014; revised August 21, 2014; accepted August 22, 2014; posted August 22, 2014 (Doc. ID 216769); published September 23, 2014
We demonstrate a single-frequency, continuous-wave, optically pumped semiconductor laser (OPSL) using an injection-locking technique. Parameters such as output coupling, injection power, and injection wavelength are investigated. With the proper parameters, the output power of the injection-locked laser exceeds the output power of its free-running condition. With 1.5 W of injection power from the master laser, over 9 W of output power is achieved at 1015 nm. © 2014 Optical Society of America OCIS codes: (140.3520) Lasers, injection-locked; (140.3570) Lasers, single-mode; (140.5960) Semiconductor lasers; (140.7270) Vertical emitting lasers. http://dx.doi.org/10.1364/OL.39.005610
Injection-locking is one of the useful techniques that have been applied on Nd:YAG [1,2], Ti:sapphire [3], and diode [4] lasers to achieve high output power while maintaining single-frequency operation. Nd:YAG lasers could provide high output power, but the emission wavelength is limited by its narrow bandwidth. Ti:sapphire lasers, on the other hand, have large gain bandwidth, but the output power is relatively low, and the pump source is relatively expensive. Optically pumped semiconductor lasers (OPSLs), or vertical external-cavity surface-emitting lasers (VECSELs), are promising semiconductor lasers that the emission wavelength can be tailored through bandgap engineering and temperature control. OPSLs are insensitive to the pump wavelength as opposed to solid-state lasers, which require the pump diodes to be temperature controlled. The external cavity enables OPSL to be operated similarly to conventional solid-state lasers for different applications. One can simply insert optical elements inside the laser cavity such as an etalon or a birefringent filter for single-frequency operation, nonlinear crystals for frequency conversion [5], and semiconductor saturable absorber mirrors (SESAMs) [6] for mode locking to generate ultra-short pulses. By way of free-space resonator, OPSLs can offer near diffraction-limited, highquality Gaussian beams, and output power is scalable by enlarging the pump spot to some extent [7]. These advantages above make OPSL a good candidate to realize high-power, single-frequency operation at tailored wavelengths in combination with the injection-locking technique. In this Letter, we demonstrate the injectionlocked operation of OPSLs and investigate how its output performance is affected by output coupling, injection wavelengths and injection power. An injection-locked laser system consists of a highpower laser (slave laser) and a lower-power singlefrequency laser (master laser). The lasers used in our system are both InGaAs-based OPSLs that have ARcoating at the lasing wavelength of 1015 nm on the gain chip. The measured pump reflectivity is 4%. The OPSL chips are bonded to 500-μm-thick CVD diamond heat spreaders and mounted on water-cooled copper heat 0146-9592/14/195610-04$15.00/0
sinks. The master laser is a linear-cavity OPSL providing up to 1.5 W of output power at 1015 nm in single frequency enforced by a birefringent filter and an etalon. The birefringent filter is a 3-mm-thick uncoated quartz plate inserted in the master laser at the Brewster’s angle. By rotating the optical axis of the birefringent filter, the emission wavelength of the master laser can be tuned from 1008 to 1022 nm, with somewhat lower output power toward the ends of the tuning range. An etalon is inserted to ensure single-frequency operation. The injection-locking is maintained using Pound–Drever–Hall technique [8,9]. As shown in Fig. 1, the injecting beam from the master laser passes through a Faraday isolator to avoid optical feedback. A 20 MHz of RF signal from the local oscillator is fed into the electro-optic phase modulator. The signal modulates the phase of the incident laser to generate sidebands. The injecting beam is modematched to the slave laser, whose TEM00 mode has the radius of 250 μm at the OPSL device. The pump to mode ratio of the slave laser is optimized to 0.72 for single transverse mode operation with M2 1.08 along horizontal axis and M2 1.04 along vertical axis. The cavity length of the slave laser is controlled by a piezoelectric transducer (PZT) that is attached on one of the mirrors of the laser. The slave laser is a ring-cavity OPSL so that the
Fig. 1. Schematic of injection-locked OPSL. LO, local oscillator; EOM, electro-optic modulator; PD, photodiode; OC, output coupler. © 2014 Optical Society of America
October 1, 2014 / Vol. 39, No. 19 / OPTICS LETTERS
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optical feedback into the master laser can be minimized. A small portion of the output beam from the slave laser is incident on a high-speed photodetector. The signal received by the photodetector contains information of frequency detuning and is then mixed with 20-MHz RF signal from the local oscillator to generate the error signal. The low-frequency component of the error signal is extracted and amplified by a servo controller and is fed back to the PZT to achieve single-frequency operation. In order to verify whether the laser is under singlefrequency operation, we observe the laser spectrum with an optical spectral analyzer. Figure 2 shows the spectra with or without the servo loop closed. Both curves are plotted on the same scale. The amplitudes are normalized to the peak value of the injection-locked signal. In freerunning mode, two lasers operate separately with the wavelength of the master laser at 1015 nm. The FWHM of the free-running laser ranges from 2 to 3 nm depending on the pump power. When the servo loop is closed, the frequency of the slave laser is locked to that of the master laser. All the power is contained in the spectral component of that of the master laser, and the components near 1017 nm vanish. The actual spectral bandwidth is less than the resolution bandwidth of the optical spectrum analyzer, 0.07 nm. Single-frequency operation is confirmed by a scanning Fabry–Perot interferometer. After the laser is injection-locked, the M2 values are 1.04 and 1.02 along the horizontal axis and the vertical axis, respectively. The beam quality remains nearly the same as the free-running slave laser. Figure 3 shows the comparison of the output powers of free-running operation and injection locked operation with 3 different output couplers (T 1%, 3%, and 5%) and 500 mW of injection power. The slave laser did not reach threshold with T 5% output coupler, indicating relatively low optical gain of OPSL devices. The ring laser operates bidirectionally under free-running condition. The total power of the free-running laser is assessed by the sum of the two outputs that is measured right after the output coupler. It is multiplied by 93%, the reflectivity of the mirror after the output coupler, for the purpose of comparison with the power of the injection-locked laser. In injection-locking operation, the laser is forced to emit unidirectionally in the same direction as the injecting laser beam. The slave laser acts as an amplifier, and
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Fig. 3. Output power of the injection-locked laser and the free-running slave laser using different output couplers. The injection-locked laser is injected by 500 mW of the master laser power at 1016 nm.
the output power of the injection-locked laser grows with the pump power. The measurement was stopped at the onset of parasitic oscillation, which can be observed on the optical spectrum analyzer. The parasitic oscillation can be also observed in the counter propagation direction by photodiode 2 where the beam leaks out from the HR mirror. Among the three output couplers used, T 1% output coupler yielded the parasitic oscillation occurs at low pump power, but the laser could be injection-locked without the onset of parasitic oscillation beyond 20 W of the pump power with T 3% and 5%. T 3% output coupler has the best output efficiency with its output power in injection-locking mode higher than in freerunning mode. Although the slave laser did not lase with T 5% output coupler, when the laser is injectionlocked, the output power grows with the increasing pump power, operating as an amplifier. The emission wavelength of a free-running OPSL is determined by the combination of the quantum well gain and the microcavity resonance at a given temperature. The emission wavelength shifts to longer wavelengths as the pump power increases since the gain structure is heated up and the bandgap of the quantum well shrinks. The free-running slave laser emits from 1014 to 1020 nm with water temperature of 15°C. For the injection-locked laser system, we investigated the output performance affected by different injection wavelengths and different injection power using the T 3% output coupler. As shown in Fig. 4, the best injection wavelength is from 1014 to 1016 nm, which is in general shorter than the emission wavelength of the free-running slave laser. Outside this range, the laser is still able to be injectionlocked, but with lower output efficiency. With the increasing pump power, the slope of output power with shorter injection wavelengths becomes flat, while that with longer injection wavelengths becomes steeper. It is a dynamic effect that the optimum injection wavelength shifts as the gain peak of the injection-locked laser shifts by a higher pump power. With injection wavelength at 1018 nm and above, the laser remains injection-locked at 25 W of pump power, whereas parasitic oscillation
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Fig. 4. Left: the output power as a function of total pump power for different injection wavelengths from 1008 to 1022 nm. Right: wavelength dependent output power of the injection-locked OPSL with incremental pump power.
starts to occur at the same pump power with the injection wavelength at 1016 nm. It is also noteworthy that more injection power is needed to maintain the injection-locking operation of OPSLs. A Nd:YAG laser can be injection locked with 40 mW of master laser power [1], but an injection-locked OPSL requires several hundreds of mW to several watts. This is probably because the saturation intensity of OPSL is much higher than that of Nd:YAG or other solid-state laser materials with typically a few to a few tens of kW∕cm2 . Our preliminary estimate based on the off-axis photoluminescence observation indicates that the saturation intensity of OPSL to be around 100 kW∕cm2 . Higher injection power extends the locking ability to a higher pumping rate. Besides, higher injection power also broadens the locking spectral range. With 1 W of injection power, the output efficiency becomes quite similar from 1014 to 1020 nm. The gain peak of the injection-locked laser shifts to longer wavelengths with higher pump powers, but with a smaller value of wavelength shift compared to the free-running condition. This can be explained by the difference in dissipated power that heats up the chip causing different wavelength shift. The injection-locked laser starts to emit photons when the pump power of the slave laser increases from zero, as the slave laser plays a role of an amplifier. The carriers, which could have been recombined through other paths such as nonradiative recombination or Auger recombination under the free-running operation, recombine radiatively through stimulated emission. Higher radiative efficiency results in not only
more extracted power from the slave laser, but also less dissipated power and lower temperature rise in the gain medium. Consequently, the preferable wavelength of injection locking is shorter compared to the free-running wavelength of the slave laser at the same pump power. Moreover, the output power of a free-running OPSL rolls over when the laser chip heats up, and the quantum well gain and microcavity resonant center misalign spectrally. The conversion efficiency of the injection-locked laser is determined by how well the gain peak of the slave laser aligns with the master laser spectrum. The gain peak of the injection-locked laser shifts less than the free-running laser providing higher output efficiency and higher rollover point. The free-running slave laser rolls over at 23 W of pump power with T 3% output coupler in Fig. 3. On the other hand, the injection-locked laser grows linearly with no sign of rollover, with its output power limited by the onset of parasitic oscillations. To calculate the dissipated power, we only consider conditions at the threshold and above the threshold: P diss T × A × P th × 1 − ηrad λp ; × 1 − ηrad las λlas
λp P inc − P th th λlas (1)
where T is transmission of the pump power at the air–chip interface and A is absorption ratio in the active region. P th and P inc are the incident pump power at the threshold and above the threshold, respectively. The pump power absorbed in DBR is neglected in the calculation. ηrad represents the radiative quantum efficiency and is the ratio between the number of emitted photons and the absorbed photons per unit time. Below the threshold, the carrier number builds up as the pump power is increased, raising the spontaneous emission rate and increasing the radiative quantum efficiency until the lasing threshold is reached. When the pump power goes above threshold, the stimulated emission dominates the photon generation process and saturates the optical gain, so ηrad becomes a larger constant number after lasing. The radiative quantum efficiency of the laser is estimated from the output performance of the free-running laser: P las P inc − P th ηdiff ;
(2)
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October 1, 2014 / Vol. 39, No. 19 / OPTICS LETTERS
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ηeff ηout ηquant ηrad ηabs :
(3)
The calculation details can be found in Ref. [10]. The differential efficiency of the free-running slave laser is 37%. With T 3% output coupler, and an estimated 1% of intracavity loss, the output coupling efficiency ηout is 75%. The quantum defect efficiency ηquant is 80% with the pump wavelength at 808 nm. Assuming that the pump absorption efficiency ηabs is 80%, the radiative quantum efficiency above the threshold is calculated to be around 80%. The radiative quantum efficiency at the threshold ηrad th is chosen to be 50%. The difference in dissipated power is nearly a constant above threshold is shown in Fig. 5(a). The difference in dissipated power could be translated into the wavelength shift through the thermal impedance of the slave laser, which we estimate to be 4 K/W for the experimental conditions here. The temperature-dependent wavelength shift Δλ∕ΔT is 0.23 nm/K. With AR-coating on the gain chip, Δλ∕ΔT is closer to the value of quantum well shifting rate (∼0.3 nm∕k) than an uncoated device by its microcavity resonance effect (with a shift rate of ∼0.1 nm∕k) is much weakened. The optimum injection wavelength is calculated according to the free-running wavelength in Fig. 5(b). For the injection-locking condition, the temperature at the gain medium is about 8.2°C
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lower, and the wavelength shift is 1.9 nm shorter, which is consistent with the fact that the best injection-locking is observed with ∼2 nm shorter wavelength than in free running mode. By monitoring the temperature at the copper heat sink, we also observe the temperature rise of the injection-locked laser is lower compared to its free-running mode at a given pump power. The output power of the injection-locked laser is further improved by lowering the water temperature to 5°C. As seen in Fig. 6, with 1.5 W of power from the master laser, the injection-locked laser generates over 9 W at 1015 nm. The maximum output power is limited by the onset of parasitic oscillation, and it does not roll over. Besides, the spectral gain of the master laser is centered at 1015 nm, and the available output power of the master laser at longer wavelength is limited. We have demonstrated a single-frequency, continuouswave, tunable wavelength, injection-locked OPSL. By using T 3% output coupler, the injection-locked laser exhibits the best output efficiency with its output power exceeding the free-running slave laser. With more extracted power, the heat generated in the gain medium is less, and consequently, the gain peak shifts in a lower rate. Over 9 W is achieved with 1.5 W of master laser power at 5°C. References 1. C. D. Nabors, A. D. Farinas, T. Day, S. T. Yang, E. K. Gustafson, and R. L. Byer, Opt. Lett. 14, 1189 (1989). 2. K. Takeno, T. Ozeki, S. Moriwaki, and N. Mio, Opt. Lett. 30, 2110 (2005). 3. E. A. Cummings, M. S. Hicken, and S. D. Bergeson, Appl. Opt. 41, 7583 (2002). 4. L. Goldberg and J. F. Weller, Appl. Phys. Lett. 50, 1713 (1987). 5. T. D. Raymond, W. J. Alford, M. H. Crawford, and A. A. Allerman, Opt. Lett. 24, 1127 (1999). 6. U. Keller and A. C. Tropper, Phys. Rep. 429, 67 (2006). 7. A. J. Maclean, R. B. Birch, P. W. Roth, A. J. Kemp, and D. Burns, J. Opt. Soc. Am. B 26, 2228 (2009). 8. R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, Appl. Phys. B 31, 97 (1983). 9. E. D. Black, Am. J. Phys. 69, 79 (2001). 10. O. G. Okhotnikov, Semiconductor Disk Lasers: Physics and Technology (Wiley-VCH, 2010), p. 328.
OPTICS LETTERS / Vol. 39, No. 19 / October 1, 2014
Injection-locked operation of an optically pumped semiconductor laser Yi-Ying Lai,1,2,* Yevgeniy Merzlyak,1,2 J. M. Yarborough,1,2 Kevin Winn,1,2 and Yushi Kaneda1,2 1
College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA 2 ERATO, Katori Innovative Space-Time Project, Tokyo 113-8656, Japan *Corresponding author: [email protected] Received July 17, 2014; revised August 21, 2014; accepted August 22, 2014; posted August 22, 2014 (Doc. ID 216769); published September 23, 2014
We demonstrate a single-frequency, continuous-wave, optically pumped semiconductor laser (OPSL) using an injection-locking technique. Parameters such as output coupling, injection power, and injection wavelength are investigated. With the proper parameters, the output power of the injection-locked laser exceeds the output power of its free-running condition. With 1.5 W of injection power from the master laser, over 9 W of output power is achieved at 1015 nm. © 2014 Optical Society of America OCIS codes: (140.3520) Lasers, injection-locked; (140.3570) Lasers, single-mode; (140.5960) Semiconductor lasers; (140.7270) Vertical emitting lasers. http://dx.doi.org/10.1364/OL.39.005610
Injection-locking is one of the useful techniques that have been applied on Nd:YAG [1,2], Ti:sapphire [3], and diode [4] lasers to achieve high output power while maintaining single-frequency operation. Nd:YAG lasers could provide high output power, but the emission wavelength is limited by its narrow bandwidth. Ti:sapphire lasers, on the other hand, have large gain bandwidth, but the output power is relatively low, and the pump source is relatively expensive. Optically pumped semiconductor lasers (OPSLs), or vertical external-cavity surface-emitting lasers (VECSELs), are promising semiconductor lasers that the emission wavelength can be tailored through bandgap engineering and temperature control. OPSLs are insensitive to the pump wavelength as opposed to solid-state lasers, which require the pump diodes to be temperature controlled. The external cavity enables OPSL to be operated similarly to conventional solid-state lasers for different applications. One can simply insert optical elements inside the laser cavity such as an etalon or a birefringent filter for single-frequency operation, nonlinear crystals for frequency conversion [5], and semiconductor saturable absorber mirrors (SESAMs) [6] for mode locking to generate ultra-short pulses. By way of free-space resonator, OPSLs can offer near diffraction-limited, highquality Gaussian beams, and output power is scalable by enlarging the pump spot to some extent [7]. These advantages above make OPSL a good candidate to realize high-power, single-frequency operation at tailored wavelengths in combination with the injection-locking technique. In this Letter, we demonstrate the injectionlocked operation of OPSLs and investigate how its output performance is affected by output coupling, injection wavelengths and injection power. An injection-locked laser system consists of a highpower laser (slave laser) and a lower-power singlefrequency laser (master laser). The lasers used in our system are both InGaAs-based OPSLs that have ARcoating at the lasing wavelength of 1015 nm on the gain chip. The measured pump reflectivity is 4%. The OPSL chips are bonded to 500-μm-thick CVD diamond heat spreaders and mounted on water-cooled copper heat 0146-9592/14/195610-04$15.00/0
sinks. The master laser is a linear-cavity OPSL providing up to 1.5 W of output power at 1015 nm in single frequency enforced by a birefringent filter and an etalon. The birefringent filter is a 3-mm-thick uncoated quartz plate inserted in the master laser at the Brewster’s angle. By rotating the optical axis of the birefringent filter, the emission wavelength of the master laser can be tuned from 1008 to 1022 nm, with somewhat lower output power toward the ends of the tuning range. An etalon is inserted to ensure single-frequency operation. The injection-locking is maintained using Pound–Drever–Hall technique [8,9]. As shown in Fig. 1, the injecting beam from the master laser passes through a Faraday isolator to avoid optical feedback. A 20 MHz of RF signal from the local oscillator is fed into the electro-optic phase modulator. The signal modulates the phase of the incident laser to generate sidebands. The injecting beam is modematched to the slave laser, whose TEM00 mode has the radius of 250 μm at the OPSL device. The pump to mode ratio of the slave laser is optimized to 0.72 for single transverse mode operation with M2 1.08 along horizontal axis and M2 1.04 along vertical axis. The cavity length of the slave laser is controlled by a piezoelectric transducer (PZT) that is attached on one of the mirrors of the laser. The slave laser is a ring-cavity OPSL so that the
Fig. 1. Schematic of injection-locked OPSL. LO, local oscillator; EOM, electro-optic modulator; PD, photodiode; OC, output coupler. © 2014 Optical Society of America
October 1, 2014 / Vol. 39, No. 19 / OPTICS LETTERS
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optical feedback into the master laser can be minimized. A small portion of the output beam from the slave laser is incident on a high-speed photodetector. The signal received by the photodetector contains information of frequency detuning and is then mixed with 20-MHz RF signal from the local oscillator to generate the error signal. The low-frequency component of the error signal is extracted and amplified by a servo controller and is fed back to the PZT to achieve single-frequency operation. In order to verify whether the laser is under singlefrequency operation, we observe the laser spectrum with an optical spectral analyzer. Figure 2 shows the spectra with or without the servo loop closed. Both curves are plotted on the same scale. The amplitudes are normalized to the peak value of the injection-locked signal. In freerunning mode, two lasers operate separately with the wavelength of the master laser at 1015 nm. The FWHM of the free-running laser ranges from 2 to 3 nm depending on the pump power. When the servo loop is closed, the frequency of the slave laser is locked to that of the master laser. All the power is contained in the spectral component of that of the master laser, and the components near 1017 nm vanish. The actual spectral bandwidth is less than the resolution bandwidth of the optical spectrum analyzer, 0.07 nm. Single-frequency operation is confirmed by a scanning Fabry–Perot interferometer. After the laser is injection-locked, the M2 values are 1.04 and 1.02 along the horizontal axis and the vertical axis, respectively. The beam quality remains nearly the same as the free-running slave laser. Figure 3 shows the comparison of the output powers of free-running operation and injection locked operation with 3 different output couplers (T 1%, 3%, and 5%) and 500 mW of injection power. The slave laser did not reach threshold with T 5% output coupler, indicating relatively low optical gain of OPSL devices. The ring laser operates bidirectionally under free-running condition. The total power of the free-running laser is assessed by the sum of the two outputs that is measured right after the output coupler. It is multiplied by 93%, the reflectivity of the mirror after the output coupler, for the purpose of comparison with the power of the injection-locked laser. In injection-locking operation, the laser is forced to emit unidirectionally in the same direction as the injecting laser beam. The slave laser acts as an amplifier, and
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Fig. 3. Output power of the injection-locked laser and the free-running slave laser using different output couplers. The injection-locked laser is injected by 500 mW of the master laser power at 1016 nm.
the output power of the injection-locked laser grows with the pump power. The measurement was stopped at the onset of parasitic oscillation, which can be observed on the optical spectrum analyzer. The parasitic oscillation can be also observed in the counter propagation direction by photodiode 2 where the beam leaks out from the HR mirror. Among the three output couplers used, T 1% output coupler yielded the parasitic oscillation occurs at low pump power, but the laser could be injection-locked without the onset of parasitic oscillation beyond 20 W of the pump power with T 3% and 5%. T 3% output coupler has the best output efficiency with its output power in injection-locking mode higher than in freerunning mode. Although the slave laser did not lase with T 5% output coupler, when the laser is injectionlocked, the output power grows with the increasing pump power, operating as an amplifier. The emission wavelength of a free-running OPSL is determined by the combination of the quantum well gain and the microcavity resonance at a given temperature. The emission wavelength shifts to longer wavelengths as the pump power increases since the gain structure is heated up and the bandgap of the quantum well shrinks. The free-running slave laser emits from 1014 to 1020 nm with water temperature of 15°C. For the injection-locked laser system, we investigated the output performance affected by different injection wavelengths and different injection power using the T 3% output coupler. As shown in Fig. 4, the best injection wavelength is from 1014 to 1016 nm, which is in general shorter than the emission wavelength of the free-running slave laser. Outside this range, the laser is still able to be injectionlocked, but with lower output efficiency. With the increasing pump power, the slope of output power with shorter injection wavelengths becomes flat, while that with longer injection wavelengths becomes steeper. It is a dynamic effect that the optimum injection wavelength shifts as the gain peak of the injection-locked laser shifts by a higher pump power. With injection wavelength at 1018 nm and above, the laser remains injection-locked at 25 W of pump power, whereas parasitic oscillation
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Fig. 4. Left: the output power as a function of total pump power for different injection wavelengths from 1008 to 1022 nm. Right: wavelength dependent output power of the injection-locked OPSL with incremental pump power.
starts to occur at the same pump power with the injection wavelength at 1016 nm. It is also noteworthy that more injection power is needed to maintain the injection-locking operation of OPSLs. A Nd:YAG laser can be injection locked with 40 mW of master laser power [1], but an injection-locked OPSL requires several hundreds of mW to several watts. This is probably because the saturation intensity of OPSL is much higher than that of Nd:YAG or other solid-state laser materials with typically a few to a few tens of kW∕cm2 . Our preliminary estimate based on the off-axis photoluminescence observation indicates that the saturation intensity of OPSL to be around 100 kW∕cm2 . Higher injection power extends the locking ability to a higher pumping rate. Besides, higher injection power also broadens the locking spectral range. With 1 W of injection power, the output efficiency becomes quite similar from 1014 to 1020 nm. The gain peak of the injection-locked laser shifts to longer wavelengths with higher pump powers, but with a smaller value of wavelength shift compared to the free-running condition. This can be explained by the difference in dissipated power that heats up the chip causing different wavelength shift. The injection-locked laser starts to emit photons when the pump power of the slave laser increases from zero, as the slave laser plays a role of an amplifier. The carriers, which could have been recombined through other paths such as nonradiative recombination or Auger recombination under the free-running operation, recombine radiatively through stimulated emission. Higher radiative efficiency results in not only
more extracted power from the slave laser, but also less dissipated power and lower temperature rise in the gain medium. Consequently, the preferable wavelength of injection locking is shorter compared to the free-running wavelength of the slave laser at the same pump power. Moreover, the output power of a free-running OPSL rolls over when the laser chip heats up, and the quantum well gain and microcavity resonant center misalign spectrally. The conversion efficiency of the injection-locked laser is determined by how well the gain peak of the slave laser aligns with the master laser spectrum. The gain peak of the injection-locked laser shifts less than the free-running laser providing higher output efficiency and higher rollover point. The free-running slave laser rolls over at 23 W of pump power with T 3% output coupler in Fig. 3. On the other hand, the injection-locked laser grows linearly with no sign of rollover, with its output power limited by the onset of parasitic oscillations. To calculate the dissipated power, we only consider conditions at the threshold and above the threshold: P diss T × A × P th × 1 − ηrad λp ; × 1 − ηrad las λlas
λp P inc − P th th λlas (1)
where T is transmission of the pump power at the air–chip interface and A is absorption ratio in the active region. P th and P inc are the incident pump power at the threshold and above the threshold, respectively. The pump power absorbed in DBR is neglected in the calculation. ηrad represents the radiative quantum efficiency and is the ratio between the number of emitted photons and the absorbed photons per unit time. Below the threshold, the carrier number builds up as the pump power is increased, raising the spontaneous emission rate and increasing the radiative quantum efficiency until the lasing threshold is reached. When the pump power goes above threshold, the stimulated emission dominates the photon generation process and saturates the optical gain, so ηrad becomes a larger constant number after lasing. The radiative quantum efficiency of the laser is estimated from the output performance of the free-running laser: P las P inc − P th ηdiff ;
(2)
where the differential efficiency ηdiff is 10
1020
free-running injection-locked
8
free-running injection-locked
1019 W avelength (nm)
7
7
Dissipated Power (W)
5612
6 4 2
1018 1017 1016 1015 1014 1013
0 0
5
10 15 20 Pump Power (W)
(a)
25
30
10
15 20 Pump Power (W)
25
(b)
Fig. 5. (a) Dissipated power calculation. (b) The central emission wavelength of the free-running laser and the calculated optimum injection wavelength.
October 1, 2014 / Vol. 39, No. 19 / OPTICS LETTERS
Output Power (W)
10
Injection-locked Free-running
8 6 4 2 0 0
5
10 15 20 25 30 Total Pump Power (W)
35
Fig. 6. Injection-locked laser output power with 1.5 W of injection power at 5°C.
ηeff ηout ηquant ηrad ηabs :
(3)
The calculation details can be found in Ref. [10]. The differential efficiency of the free-running slave laser is 37%. With T 3% output coupler, and an estimated 1% of intracavity loss, the output coupling efficiency ηout is 75%. The quantum defect efficiency ηquant is 80% with the pump wavelength at 808 nm. Assuming that the pump absorption efficiency ηabs is 80%, the radiative quantum efficiency above the threshold is calculated to be around 80%. The radiative quantum efficiency at the threshold ηrad th is chosen to be 50%. The difference in dissipated power is nearly a constant above threshold is shown in Fig. 5(a). The difference in dissipated power could be translated into the wavelength shift through the thermal impedance of the slave laser, which we estimate to be 4 K/W for the experimental conditions here. The temperature-dependent wavelength shift Δλ∕ΔT is 0.23 nm/K. With AR-coating on the gain chip, Δλ∕ΔT is closer to the value of quantum well shifting rate (∼0.3 nm∕k) than an uncoated device by its microcavity resonance effect (with a shift rate of ∼0.1 nm∕k) is much weakened. The optimum injection wavelength is calculated according to the free-running wavelength in Fig. 5(b). For the injection-locking condition, the temperature at the gain medium is about 8.2°C
5613
lower, and the wavelength shift is 1.9 nm shorter, which is consistent with the fact that the best injection-locking is observed with ∼2 nm shorter wavelength than in free running mode. By monitoring the temperature at the copper heat sink, we also observe the temperature rise of the injection-locked laser is lower compared to its free-running mode at a given pump power. The output power of the injection-locked laser is further improved by lowering the water temperature to 5°C. As seen in Fig. 6, with 1.5 W of power from the master laser, the injection-locked laser generates over 9 W at 1015 nm. The maximum output power is limited by the onset of parasitic oscillation, and it does not roll over. Besides, the spectral gain of the master laser is centered at 1015 nm, and the available output power of the master laser at longer wavelength is limited. We have demonstrated a single-frequency, continuouswave, tunable wavelength, injection-locked OPSL. By using T 3% output coupler, the injection-locked laser exhibits the best output efficiency with its output power exceeding the free-running slave laser. With more extracted power, the heat generated in the gain medium is less, and consequently, the gain peak shifts in a lower rate. Over 9 W is achieved with 1.5 W of master laser power at 5°C. References 1. C. D. Nabors, A. D. Farinas, T. Day, S. T. Yang, E. K. Gustafson, and R. L. Byer, Opt. Lett. 14, 1189 (1989). 2. K. Takeno, T. Ozeki, S. Moriwaki, and N. Mio, Opt. Lett. 30, 2110 (2005). 3. E. A. Cummings, M. S. Hicken, and S. D. Bergeson, Appl. Opt. 41, 7583 (2002). 4. L. Goldberg and J. F. Weller, Appl. Phys. Lett. 50, 1713 (1987). 5. T. D. Raymond, W. J. Alford, M. H. Crawford, and A. A. Allerman, Opt. Lett. 24, 1127 (1999). 6. U. Keller and A. C. Tropper, Phys. Rep. 429, 67 (2006). 7. A. J. Maclean, R. B. Birch, P. W. Roth, A. J. Kemp, and D. Burns, J. Opt. Soc. Am. B 26, 2228 (2009). 8. R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, Appl. Phys. B 31, 97 (1983). 9. E. D. Black, Am. J. Phys. 69, 79 (2001). 10. O. G. Okhotnikov, Semiconductor Disk Lasers: Physics and Technology (Wiley-VCH, 2010), p. 328.
