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Letter

Vol. 40, No. 22 / November 15 2015 / Optics Letters

Electro-optically spectrum tailorable intracavity optical parametric oscillator H. P. CHUNG,1 W. K. CHANG,1 C. H. TSENG,1 R. GEISS,2 T. PERTSCH,2

AND

Y. H. CHEN1,*

1

Department of Optics and Photonics, National Central University, Jhongli 320, Taiwan Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany *Corresponding author: [email protected]

2

Received 25 June 2015; revised 25 September 2015; accepted 25 September 2015; posted 25 September 2015 (Doc. ID 243734); published 2 November 2015

We report a unique, pulsed intracavity optical parametric oscillator (IOPO) whose output spectrum is electrooptically (EO) tailorable based on an aperiodically poled lithium niobate (APPLN) working simultaneously as an optical parametric gain medium and an active gain spectrum filter in the system. We have successfully obtained from the IOPO the emission of single to multiple narrow-line signal spectral peaks in a near-infrared (1531 nm) band simply by electro-optic control. The power spectral density of the EO tailored signal can be enhanced by up to 10 times over the original (nontailored) signal. © 2015 Optical Society of America OCIS

codes:

(230.2090)

Electro-optical

devices;

(190.4970)

Parametric oscillators and amplifiers. http://dx.doi.org/10.1364/OL.40.005132

A pulsed (nanosecond) optical parametric oscillator (OPO) has been an advantageous light source for many applications, such as remote sensing, biomedical measurements, and spectroscopy. Narrow-line operation of the OPO is essential to build a reliable and high-resolution measurement system for these applications. However, in contrast to a continuous-wave (cw) OPO, a pulsed OPO is usually characterized by broader spectral bandwidth (on the order of a few nanometers in the near infrared) because of the high parametric gain. To reduce the spectral linewidth of an OPO, one has resorted to the installation of a wavelength selective element in the oscillator system [1–3]. These intracavity elements are, however, separated from the parametric gain medium and, therefore, will introduce extra loss to the system. Besides, although the OPO signal can potentially be tuned over the gain bandwidth by these elements, it relies on a slow mechanical tuning mechanism. Moreover, these elements usually select one wavelength at one time in the parametric gain bandwidth for the oscillation and lack an active mechanism for performing spectral tailoring of the OPO signal for, as an example, multi-spectral-line generation. It is thus desirable to develop an OPO whose signal spectrum can be actively tailored to produce narrowband single to multiple spectral lines. Such a unique OPO has been first demonstrated in a three-sec0146-9592/15/225132-04$15/0$15.00 © 2015 Optical Society of America

tion, monolithically cascaded, periodically poled lithium niobate (PPLN) device externally pumped by a flash-lamp-pumped, Qswitched Nd:YAG laser [4]. In that device scheme, a PPLN optical parametric gain medium (OPGM) is sandwiched between two PPLN electro-optic (EO) polarization-mode converters (PMCs), which work as a pair of active spectral filters [5] to tailor the signal oscillation gain spectrum. However, such a device works by relying on three temperature controllers and two voltage supplies to control the lineshape functions of the two PPLN EO PMCs and their overlap with the PPLN OPGM gain spectrum, which turns out to be a complex system. In this work, we further designed and constructed an electro-optically spectrum tailorable (EOST) OPO based on the aperiodic optical superlattice (AOS) technique [6]. This technique is used to integrate the key device functionalities, a PPLN OPGM and an aperiodically poled lithium niobate (APPLN) EO PMC, in a monolithic LiNbO3 crystal to implement such an OPO. This novel scheme allows one to operate the device with a uniform electric field and temperature. The monolithic integration design of the device also allows it to work in a diode-pumped Nd3 laser in which an intracavity OPO (IOPO) is built. Because of these favorable features, the EOST OPO developed in this work is a more compact, simple, and reliable system. The core device of the built EOST OPO integrates the functionalities of dual quasi-phase-matching (QPM) devices, a PPLN OPGM and an APPLN EO PMC. An EO QPM PMC, such as a PPLN EO PMC, can perform narrowband (∼2 nm cm at 1.5-μm band [5]) conversion between extraordinary (e) and ordinary (o) polarization modes of a wave at a specific (phase-matched) wavelength. It can thus act as a notchfilter-like wavelength selective element when it works inside a PPLN OPO cavity. This is because the PPLN OPGM generates/amplifies only the e-polarized waves and provides no gain to those converted to the o-polarized waves in which only those waves outside the conversion (notch) bandwidth of the PPLN EO PMC but within the OPO bandwidth can build up in the cavity eventually. However, it was shown in Ref. [4] that PPLN EO PMCs rely on using a high electric field (on the order of 1 kV/mm) and a proper temperature to broaden the notch bandwidth and move the notch position, respectively, to form a desired filtering spectrum. To implement a genuine EOST

Letter

Vol. 40, No. 22 / November 15 2015 / Optics Letters

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equations derived for describing waves interacting in such a dual-function QPM device. Consider an e-polarized, 1064-nm pump wave incident along the crystallographic x axis of a z-cut LiNbO3 crystal that consists of a sequence of N crystal-domain blocks, each with a thickness of Δx and a domain polarity of either 1 or −1 denoting the z or −z crystal orientation of the block, respectively. The coupled mode equations govern the optical parametric downconversion (OPDC), and, simultaneously, the EO polarization-mode conversion of the OPDC signal in such a crystal can be expressed as Fig. 1. Calculated transmission spectra of an APPLN EO PMC for an e-polarized input wave (of flat spectrum) after one cavity transit at E y = (a) 250 V/mm (solid line) and (b) 400 V/mm (solid line). The dashed line represents the calculated signal spectrum of a typical pulsed PPLN IOPO (see text).

d E s;o x ω  −i s sxκeo1 E s;e xe iΔkeo x − κeo2 E s;o x; c 0 ns;o dx d E s;e x ω  −i s sxκ eo1 E s;o xe −iΔkeo x c 0 ns;e dx  d 33 E p;e xE i;e xe −iΔkop x ;

OPO working with a lower (and uniform) electric field and without the involvement of temperature tuning, which is slow, we first introduce an APPLN EO PMC into an OPO system. The working concept of applying an APPLN EO PMC in this study can be illustrated by Fig. 1, which shows the calculated transmission spectra of an APPLN EO PMC after one (cold)cavity transit for an e-polarized input wave initially with a flat spectrum spanning over the designated calculation region (1528–1534 nm) at different applied electric fields at 36 °C. This sample APPLN device is designed to have high transmittance for e waves at 1530, 1531, and 1532 nm at a field of 250 V/mm applied along the crystallographic y axis [solid line in Fig. 1(a)]. The calculated signal spectrum of a pulsed OPO with a 3-cm-long, 29.8-μm period PPLN OPGM intracavity pumped by a 3 MW∕cm2 , 1064-nm laser is also shown for reference (dashed gray line). Such a PPLN IOPO has been studied in our lab and can produce a signal wave (at 1531 nm) with a bandwidth on the order of 1 nm under 7–10 cavity transits for the signal buildup, providing a good basis for the design of the EOST OPO developed in this work (using the APPLN technique; see below). Because a sharp and highly transmitted peak in the PMC spectrum (1531 nm) falls in the central parametric gain [see Fig. 1(a)], it is reasonable to anticipate the OPO oscillating on a single narrowed (in relation to the original signal spectrum) spectral peak when operated with the APPLN EO PMC. Uniquely, more interesting signal spectra can be expected from such a system via the EO tuning of the APPLN device [e.g., three narrowed spectral peaks can potentially be generated when the same APPLN is operated at 400 V/mm; see the corresponding PMC transmission spectrum in solid line in Fig. 1(b)]. This is not surprising, as the conversion spectrum of an EO QPM PMC is a sensitive function of the driving field in its phase-matching bandwidth. Since an APPLN EO PMC can provide multiple (closely spaced) phasematched channels [7] in a desired band, manipulation of the output spectrum of the device can then be done with great freedom. Note that those transmission spectra of the APPLN EO PMC device shown in Fig. 1 are calculated in the nonresonant mode (i.e., no parametric gain and cavity oscillation are involved) only for the rudimentary illustration. To analytically predict the output performance of the EOST IOPO using the proposed APPLN device, we have developed a calculation model based on the simultaneous solution of the coupled mode

d E i;e x ω  −i i sxd 33 E p;e xE s;e xe −iΔkop x ; c 0 ni;e dx d E p;e x ωp  −i sxd 33 E s;e xE i;e xe iΔkop x ; c 0 np;e dx

(1)

where i is the imaginary unit; Ex is the field amplitude envelope; ω is the angular frequency; n is the refractive index [8]; the subscripts s, i, and p and o and e denote quantities associated with the signal, idler, and pump waves and the ordinary and extraordinary polarization modes, respectively; c 0 is the speed of light in a vacuum; sx  1 is a sign function denoting the z domain polarity of the crystal block at position x; d 33 is the accessed nonlinear coefficient; Δk eo  k s;o − ks;e and Δkop  k p;e − ks;e − ki;e are the wave-vector mismatches of the EO polarization-mode conversion and the OPDC processes, respectively; and κ eo1  −1∕2n2s;o n2s;e r 42 E y and κ eo2  −1∕2n4s;o r 22 E y are the coupling coefficients of the EO polarization-mode conversion process, where r 42 and r 22 are the relevant Pockels coefficients and E y is an external electric field applied along the y axis of the crystal. The output signal of the EOST OPO, simultaneously gained and EO manipulated in the APPLN during its buildup, can then be simulated by iteratively solving Eq. (1). To construct an optimal domain structure for the core device, we adopted the AOS technique and built an algorithm based on the simulated annealing (SA) method [9] in the calculation model developed above. In this calculation, an objective function (OF) has been used to guide the SA algorithm to best construct the domain structure sx of a LiNbO3 crystal, given by OF  jηop;0 λs  − ηop λs jwop λs   X M jηeo1;0 λs;α  − ηeo1 λs;α jweo1 λs;α  ; 

(2)

α1

where λs is the phase-matched signal wavelength, ηop;0 λs  and ηop λs  ≡ jE s;e px N j2 ∕jE p;e 0j2 (where px N represents a number of p cavity transits of the wave evolution) are the target and calculated conversion efficiencies of the OPO, wop λs  is the efficiency weighting factor for the OPDC process, M is the number of the EO QPM polarization-mode conversion processes, ηeo1;0 λs; α and ηeo1 λs;α  ≡ jE s;o px N j2 ∕jE s;e px N j2  jE s;o px N j2 jλs;α are the target and calculated efficiencies of

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Vol. 40, No. 22 / November 15 2015 / Optics Letters

the αth EO conversion process for a designated wavelength λs; α in the signal oscillation bandwidth, and weo1 λs; α is the efficiency weighting factor for the αth EO conversion process. In this work, we aim to construct an APPLN device that works as a novel OPGM whose signal gain spectrum is electrooptically controllable for oscillating single to multiple narrowline spectral peaks in a laser cavity. In design, we have set the objective function to guide the algorithm to implement three EO QPM polarization-mode conversion processes in the signal gain bandwidth, with M  3 and λs; α  1530.8, 1531.2, and 1531.5 nm (an optimal set) for α  1, 2, and 3, respectively, being used in Eq. (2). The device is designed for 36 °C. The use of M  3 in this work was found to be satisfactory in calculating an sx for the APPLN device to build a proposed EOST IOPO. The corresponding efficiency weighting factors weo1 λs; α for the three conversion processes were tuned between 0 and 1 individually to find an optimal APPLN domain structure sx. The output spectra of each tuning on weo1 were simulated and examined as a function of E y varied from 0 to 600 V/mm at a step of 50 V/mm. Several different sets of λs; α have been tested in the calculation to ensure an optimal result. Following this methodology, an sx that works to the production of one, two, and five narrow spectral peaks (FWHM ∼0.1 nm) with the IOPO operated at E y  200, 400, and 500 V/mm (in contrast to the original ∼1 nm spectral width at E y  0 V∕mm), respectively, has been successfully calculated. Figure 2(b) shows the simulated output spectrum of the IOPO with the calculated APPLN at E y  200 V∕mm under 3 MW∕cm2 pump intensity and 13 cavity roundtrips (RTs) for signal buildup. Taking this operating condition for the generation of highly narrowed single spectral peaks as an example, we found that the electro-optically tailored spectra can still be well retained with the IOPO using the calculated APPLN for a range of the pump intensities (2–3.5 MW∕cm2 ) and cavity RTs (10–15 times), as shown in Fig. 2. This implies that an EO PMC spectrum of the present design can work for a signal bandwidth tolerance of ∼0.2 nm (∼26 GHz). The calculated binary ( 1) sequence sx is then translated into the domain structure of a LiNbO3 crystal composed of N  7500 domain blocks, with each having a thickness of Δx  4 μm, forming an APPLN of a total length of 30 mm. We then fabricated the APPLN in a 30-mm-long, 1-mm-wide, and 0.5-mm-thick z-cut LiNbO3 crystal by using the standard electric-field poling technique. Both of the end (x) faces of the APPLN crystal were optically polished and had an anti-reflection coating for wavelengths at 1064 and 1531 nm. The two y faces of the crystal were sputtered with NiCr alloy serving as the electrodes where E y was applied. Figure 3 shows the schematic arrangement of the EOST OPO constructed in a diode-pumped, EO Q-switched Nd:YVO4 laser using the fabricated APPLN device. The APPLN device was mounted in a (copper-based) crystal oven that was temperature controlled at 36 °C. The laser gain medium is a 9-mm-long, a-cut 0.3-at. % Nd:YVO4 crystal with 3 mm × 3 mm clear aperture. The pump laser is an 809-nm fiber-coupled diode laser. The 1064-nm laser oscillates in a cavity consisting of a 15-cm radius-of-curvature meniscus dielectric mirror (designated M1) and a plane–plane dielectric mirror (designated M2). Mirror M1 was coated to be highly transmitting (T ∼ 93%) at 809 nm and highly reflective at 1064 and 1510–1560 nm (R ∼ 99.8% and >99.4%, respectively), while mirror M2 was coated to be

Letter

Fig. 2. Simulated output spectra of the IOPO with the calculated APPLN at E y  200 V∕mm under pump intensity of 2, 3, and 3.5 MW∕cm2 and 15, 13, and 10 cavity roundtrips, respectively.

highly reflective (R ∼ 99.6%) at 1064 nm and highly transmitting (T > 99.3%) at 1510–1560 nm. The OPDC signal produced from the intracavity pumped APPLN device oscillates in a resonator formed by mirror M1 and an output coupling mirror (designated M3). M3 is a plane–plane dielectric mirror with ∼93% reflectance at 1510–1560 nm. A singly resonant oscillator is formed since the mid-IR idler wavelength (3488 nm) will be highly attenuated by the BK7 mirrors. The laser Q-switch is a homemade PPLN EO Bragg cell [10] (10 mm long, 9 mm wide, and 0.5 mm thick) placed downstream from the APPLN device. In operation, we drove the PPLN EO Bragg cell with a 170-V, 300-ns voltage pulse train at 1 kHz to repetitively Q-switch the 1064 nm laser. The solid red curve shown in Fig. 4(a) is the measured IOPO signal spectrum at E y  0 V∕mm (referred to as the passive mode) at a diode pump power of 5.1 W, which coincides best with that calculated (dashed blue curve) based on the design parameters (3 MW∕cm2 1064-nm pump and 10 cavity-transit signal buildup). The electro-optically tailored signal spectra of the IOPO were then measured at this diode pump power and compared with the simulation results. Figures 4(b)–4(d) show the measured (solid red lines) and simulated (dashed blue lines) output signal spectra of the IOPO when operated at E y  200, 400, and 500 V/mm, respectively. In the intermediate values of the three applied electric fields, the output spectrum evolves from single to two and then to multiple spectral peaks (not shown). Some closely spaced spectral peaks were not well resolved in Fig. 4(d) due to the finite resolution (0.06 nm) of the spectral analyzer (Agilent 86142B). We suspect that the slight wavelength shift between the calculated and measured spectra could originate from the existing but weaker z component of the electric field (established between the y surface electrode where the voltage is applied and the copper oven where the −z surface of the sample is in contact with) that induces the refractive index modulation in the APPLN [11]. The numbers of the cavity transits for the signal buildup used in the simu-

Fig. 3. Schematic arrangement of the EOST APPLN OPO constructed in a diode-pumped, EO Q-switched Nd:YVO4 laser.

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Vol. 40, No. 22 / November 15 2015 / Optics Letters

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Fig. 5. Measured signal pulse energies (solid circles) and widths (diamonds) versus the applied electric fields used in the IOPO for the production of the signal spectra shown in Fig. 4.

Fig. 4. Measured (solid lines) and simulated (dashed lines) output signal spectra of the EOST IOPO when the APPLN is operated at E y = (a) 0, (b) 200, (c) 400, and (d) 500 V/mm, respectively.

lation for the best fit of the three measured spectra were 13, 11, and 11, respectively. These cavity transit numbers are within the working range of the device (see Fig. 2) and are different for different spectral generations, which can be attributable to the different gain spectrum suppression (tailoring) situations. We obtained from the IOPO the emission of a narrow-line signal spectral peak [see Fig. 4(b)] with a spectral width (FWHM) narrowed by a factor of ∼13 and a peak intensity enhanced by a factor of ∼4 in contrast to the system in passive mode. Figure 5 shows the measured signal pulse energies and widths versus the applied electric fields used in the IOPO for the production of the signal spectra shown in Fig. 4. The measured peak-to-peak intensity fluctuation of the output signal pulses was

Electro-optically spectrum tailorable intracavity optical parametric oscillator.

We report a unique, pulsed intracavity optical parametric oscillator (IOPO) whose output spectrum is electro-optically (EO) tailorable based on an ape...
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