Mid-IR optical amplification and detection using quantum cascade lasers Dingkai Guo,1,* Xing Chen,1 Liwei Cheng,1 Alexey Belyanin,2 and Fow-Sen Choa1 1

Department of Computer Science and Electrical Engineering, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA 2 Department of Physics and Astronomy, Texas A&M University, College Station, Texas 77843, USA * [email protected]

Abstract: Amplification and detection characteristics of mid-infrared quantum cascade lasers (QCLs) are studied. The QCL amplifier has an adjustable bandwidth and tunable gain peak to function as a tunable mid-IR filter. By biasing the QCL slightly below its threshold, we demonstrated more than 11dB optical gain and over 28dB electrical gain at specified wavelengths. In the electrical gain measurement process, the resonant amplifier also functioned as a detector. Mid-IR amplification and detection can be achieved using the same material for the laser source. This indicates that intersubband based gain materials can be ideal candidates for mid-IR photonic integrations. ©2013 Optical Society of America OCIS codes: (140.3280) Laser amplifiers; (140.5965) Semiconductor lasers, quantum cascade.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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#188793 - $15.00 USD Received 9 Sep 2013; revised 7 Nov 2013; accepted 29 Nov 2013; published 5 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030545 | OPTICS EXPRESS 30545

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1. Introduction Quantum cascade lasers (QCLs) [1, 2] are becoming well-established laser sources that can operate from mid-infrared to terahertz wavelengths. These wavelength ranges have great application potential in the field of chemical and environmental sensing, medical and security imaging, and free space communications [3–6]. However, even with good progress in the past [7], optical amplification and filtering functions in the mid-IR wavelength range still remain under-developed. In the near-IR region, semiconductor laser diodes have been proposed and demonstrated to function as tunable active filters as well as photo-detectors using their below threshold lasing resonance [8, 9]. In the mid-IR region, QCLs have also been reported recently as a slave single pass amplifier [10] and as a broadband mid-IR photo-detector at low voltage bias [11–13]. In this paper, we demonstrate the mid-IR filtering, amplification, and detection by exploiting resonant gain of QCLs. The QC gain material itself is used as an active filter/amplifier and narrow band photo-detector with enhanced responsivity. Since the QCL gain material is also lattice-matched to InP, it becomes an excellent candidate for midIR photonic integration and miniaturization. 2. Theoretical calculations A QCL resonant amplifier uses feedback from its resonant cavity to achieve narrow-band regenerative amplification and optical filtering. The resonant cavity can be a Fabry-Perot cavity, a distributed feedback (DFB) cavity, or a distributed Bragg reflector (DBR) cavity. When a QCL is biased below its lasing threshold but above the transparency point, resonant photons build up in response to the internal gain and spontaneous noise. When an external mid-IR coherent source is injected into this cavity, the coherent photons will be amplified by the same resonant gain and compete with other amplified spontaneous emission (ASE) noise photons. Because this resonant gain is a function of carrier density, changing the carrier density by changing the bias current can change the amplification gain and filtering profile. The signal gain G can be described by the following equation [14, 15]: G=

k 1  1   ΓvG g c ( N 0 − N t + ΔN ) − 2 τ   p

2 2

2   1  + Γ Δ + − Ω v g N α ω (    0 ) G c     2

(1)

where κ ( = c/2nL) is the proportionality coefficient, Γ( = NpΓp) is the total confinement 2   4π e 2 z21 is the gain cross section, and N0, Nt and ΔN are the carrier factor, g c  =  ε nλ ( 2γ ) L  p   0 density, transparency carrier density and light induced carrier density change respectively. Here, ω is the external light frequency, and Ω0 is the QCL cavity center frequency. The QCL has a cavity length of 3 mm, an active region thickness of 3μm and width of 12μm. The cavity

#188793 - $15.00 USD Received 9 Sep 2013; revised 7 Nov 2013; accepted 29 Nov 2013; published 5 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030545 | OPTICS EXPRESS 30546

waveguide loss and mirror loss is assumed to be 8.5 cm−1 and 2.2 cm−1 respectively for all the calculation followed. Other quantities and their values are listed in Table 1. Table 1. Parameters used in the QCL amplification calculation Symbol gc τp α

Quantity Cross section gain constant Photon lifetime QCL α factor

Calculated/Measured Value 1.4 × 10−17 m2 10 ps −0.5

c n Γp Np vG Jth τs β Nt

Light speed in vacuum Refractive index Confinement factor of one period Number of Stages in QCL Group velocity Threshold current density Carrier lifetime below threshold Spontaneous emission coupling ratio Transparency carrier density

3 × 108 m/s 3.18 1.5% 30 9.4 × 107 m/s 0.42 kA/cm2 20 ps 10−4 2 × 1019 /m2

First, the amplification characteristics are calculated under low-input power conditions (coupled optical power less than −45dBm) where gain saturation does not occur. The calculated results for the signal gain, G, against the frequency detuning from the central frequency are shown in Fig. 1(a). By increasing the QCL bias current from 70% to 99% of the lasing threshold (Ith), the peak resonant gain increases from around 0 dB to almost 35 dB. This optical gain is internal and does not include the coupling loss in and out of the amplifier cavity. As the input power increases, the saturation of the mode gain becomes pronounced, and the effective refractive index changes. This is because the increased input power increases stimulated emission and decreases injected carriers in the active layer. When the carriers decrease, this causes both gain saturation and an increase in the refractive index. The calculated cavity-coupled input power dependency of the gain spectra are shown in Fig. 1(b). QCL amplifier does not suffer from serious asymmetric saturation profile due to its small Henry’s α factor compared with NIR laser amplifier [15, 16]. In both unsaturated and saturated cases calculated above, we assumed that the QCL cavity temperature did not change in response to the external source since laser mount with good heat extraction was used in our experiment. Because the Fermi level of a laser gain medium is a function of carrier number, changing the number of carriers can effectively change the terminal voltage of the amplifier. Thus, by measuring the temporal variation in the amplifier terminal voltage, we can monitor the carrier dynamics and, correspondingly, field dynamics. The relation between generated junction voltage V and carrier density can be described by [17]: V = mVT

ΔN N0

(2)

where VT = KT/q is the thermal voltage and m is a constant relates to the QCL band structure.

#188793 - $15.00 USD Received 9 Sep 2013; revised 7 Nov 2013; accepted 29 Nov 2013; published 5 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030545 | OPTICS EXPRESS 30547

Fig. 1. (a) The amplifier gain profile vs. the input signal frequency detuning from the amplifier central frequency under low input power (cavity-coupled power less than −45dBm). The biased current through the amplifier increases from 0.7Ith to 0.99Ith. (b) The optical gain profile vs. the frequency detuning under different cavity-coupled input power from −45dBm to −10dBm when the QCL amplifier is biased just below threshold (0.99Ith). The amplifier starts to saturate when the cavity-coupled input power is higher than ~−45dBm.

3. Experiment The QCL used in the experiment has an active region grown by molecular beam epitaxy (MBE) on an n-type InP substrate (n = 2 × 1017cm−3). The active layer contains In0.53Ga0.47As/In0.52Al0.48As superlattice with a thickness of around 3µm and is sandwiched by 0.5 μm thick InGaAs layers from both bottom and top sides. The layer sequences for the injector/active region are as follows (in Angstroms, barrier layers in bold, and doped layers are underlined): 34 14 33 13 32 15 31 19 29 23 27 25 27 44 18 9 57 11 54 12 45 25 (λ≈7.9 μm). The doping level in the injector region is n = 2 × 1017cm−3. The MBE grown sample was then sent into a metal organic chemical vapor deposition (MOCVD) reactor for n-InP cover growth. The cap layers include a 2.5µm thick low doped (1x1017cm−3) upper waveguide cladding layer, a 1µm thick high doped (1x1019cm−3) plasmon-enhanced

#188793 - $15.00 USD Received 9 Sep 2013; revised 7 Nov 2013; accepted 29 Nov 2013; published 5 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030545 | OPTICS EXPRESS 30548

confinement layer, and 100 Å heavily doped (>1019cm−3) contact layer. After the cover growth, the sample followed a standard buried-heterostructure (BH) fabrication process [18] to form the 12 μm wide waveguide structure with both photonic and electronic confinements. Ti/Au was used for top and bottom metallization. The samples were then cleaved and scribed down to 3mm long and 0.5 mm wide individual QCL devices. One side of the laser facet was HR coated to achieve a lower threshold [19]. Two QCLs (QCL1 and QCL2) were then mounted epi-down with indium solder on copper heat sinks and then mounted into liquid nitrogen (LN2) dewars.

BS

Fig. 2. Experimental set-up for QCL as a resonant optical amplifier. Two QCL devices with dimension of 3mm long and 500μm wide were placed into LN2 dewars. After recording the QCL2 optical spectra under different bias current, the FTIR was then replaced by a MCT detector for optical power measurement.

The experimental setup is shown in Fig. 2. Both lasers were cooled by LN2 to 77K to achieve continuous wave (CW) mode operation or higher optical output power. QCL1 (Transmitter) was operated under pulse mode and above threshold (10KHz, 50% pulse duty cycle, and ~8V), the emission spectrum of this Fabry-Perot laser is shown in the Fig. 3(a). It was then mounted with an external grating aligned in the Littrow configuration and the output of the external cavity (the grating zero order output) was then sent through a 50/50 beamsplitter and well coupled into the QCL2 cavity. QCL2 (Receiver) was biased with a DC current through a bias-T, and the received AC electrical signal was taken out from the bias-T to a RF spectrum analyzer. The reflected QCL2 beam from the beamsplitter was sent into a FTIR for MIR spectra measurement. One shall notice that the FTIR received reflection beam from QCL2 will contain the photons directly reflected from the QCL2 facet and the photons produced from the resonant gain of the QCL2 cavity. In our measurement, QCL1 was tuned to a stable wavelength at 1289.1 cm−1. QCL2 has a lasing threshold of 152mA. Figure 3(a) shows the measured FTIR spectra of the QCL2 output when its bias is at 138mA (~0.9Ith) and 200mA (~1.3Ith). When QCL2 is operated above threshold with a bias current from 160mA to 210mA, the output wavelength can vary from 1287.6 cm−1 to 1276.2 cm−1 due to thermal effects as shown in Fig. 3(b). When the QCL2 bias current is ~140mA (below threshold) the gain peak of its electroluminescence (EL) is aligned with the QCL1 output wavelength. Since mid-IR isolator is not available to us at this moment all our results are obtained without using isolators. The reflection did not have significant influence over the QCL1 lasing wavelength, which may because of that the short carrier lifetime and small α factor of the Mid-IR QCLs compared with that of the NIR laser transmission/receiving system [15, 20].

#188793 - $15.00 USD Received 9 Sep 2013; revised 7 Nov 2013; accepted 29 Nov 2013; published 5 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030545 | OPTICS EXPRESS 30549

Fig. 3. (a) Measured FTIR spectra of QCL1 when it is pulsed at 10KHz and biased at ~8V, the main peak of its spectra is at 1289.1 cm−1; measured QCL2 electroluminescence when it is biased near 90% of its lasing threshold, and QCL2 lasing wavelength when it is biased around 200mA. (b) The truncated output spectra of QCL2 when its bias current increases from 160mA to 210mA. The main output wavelength shifted from 1287.6 cm−1 to 1276.2 cm−1 due to current induced thermal effect.

To observe amplified signals the FTIR was replaced by an MCT detector. QCL1 was modulated at 10 kHz with an average power of 1mW. By gradually increasing the QCL2 current from 0mA to 250 mA, optical amplification can be observed, which occurred right below the QCL2’s threshold. The blue trace of Fig. 4(a) is the reflected signal from QCL2 detected by the MCT detector when QCL2 is biased at very low current. When QCL2 bias is near 125mA, the blue trace amplitude starts to increase. It reached its maximum when the QCL2 bias is close to 150mA as recorded as the red trace. An optical gain of more than 11 dB is observed as shown in Fig. 4(a). It should be noticed that the real optical gain should be much higher than 11dB. Since initially when QCL2 bias is low, the resonant gain is very small. The blue trace detection is dominated by the reflected power from QCL2 front facet and probably contains very little power generated by the resonant cavity.

#188793 - $15.00 USD Received 9 Sep 2013; revised 7 Nov 2013; accepted 29 Nov 2013; published 5 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030545 | OPTICS EXPRESS 30550

Fig. 4. (a) The data extracted from the oscilloscope connecting to the QCL1 pulse generator, current probe and MCT detector. The blue trace is captured when QCL2 is at a bias below 125mA; the red trace is captured when QCL2 is biased near 150mA (near its Ith). Both traces are the voltage signal detected by the MCT detector. The detected optical signal is greatly increased in the latter case. (b) Blue line: measured receiving power from the RF spectrum analyzer as a function of receiver bias; Green line: QCL2 light-current relationship

To estimate the actual gain with our current device setup, we looked at the 10 kHz electrical output from the RF spectrum analyzer. The detected electrical power actually represents the photon number or the optical power in the laser cavity. As shown in Fig. 4(b), the detected 10kHz electrical power signal displayed at the spectrum analyzer is changing with QCL2 gain. The detected electrical signal power reaches its maximum when QCL2 is biased near 150 mA, right below the QCL2’s threshold. The detected signal power is more than 28dB higher compared to the case of low bias. The QCL2 gain peak is apparently aligned with the QCL1 output wavelength as discussed above. Upon further increasing the bias current, QCL2 started to lase and its emitting wavelength shifted toward longer wavelengths. The QCL2 gain peak moved away from the QCL1 wavelength and the electrical output signal quickly dropped even with increasing bias. The fast changing nature of output power is a combined result of both wavelength mis-alignment and bias current variation. In the future setup, a resonant cavity that can pass both sides and with very low coupling loss to the waveguide shall be designed and fabricated. #188793 - $15.00 USD Received 9 Sep 2013; revised 7 Nov 2013; accepted 29 Nov 2013; published 5 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030545 | OPTICS EXPRESS 30551

4. Conclusion A mid-infrared resonant optical amplifier based on intersubband QC structure is demonstrated. The device functions as an optical filter, an optical amplifier, and a photodetector at the mid-IR wavelength range, using the same QC gain material. A signal gain of 28 dB was demonstrated. This work may potentially open the door to photonic integration for more complex and high-performance mid-IR integrated devices. Acknowledgments The authors thank Yue Hu and Robert J. Weiblen for useful discussions.

#188793 - $15.00 USD Received 9 Sep 2013; revised 7 Nov 2013; accepted 29 Nov 2013; published 5 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030545 | OPTICS EXPRESS 30552

Mid-IR optical amplification and detection using quantum cascade lasers.

Amplification and detection characteristics of mid-infrared quantum cascade lasers (QCLs) are studied. The QCL amplifier has an adjustable bandwidth a...
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