1 October , Glenn W. Baxter a, Iain T. McKinnie b. Received 5 June 2000; accepted 2 August 2000

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1 1 October 2000 Optics Communications 184 (2000) 225±230 Single-mode visible and mid-infrared periodically poled lithium niobate optical parametric oscillator ampli ed in perylene red doped poly(methyl methacrylate) Philip Schlup a, *, Glenn W. Baxter a, Iain T. McKinnie b a Department of Physics, University of Otago, P.O. Box 56, Dunedin, New Zealand b Coherent Technologies Inc., P.O. Box 7488, Boulder, CO 80306, USA Received 5 June 2000; accepted 2 August 2000 Abstract We have demonstrated a simple grazing-incidence optical parametric oscillator (OPO) based on periodically poled lithium niobate (PPLN) capable of generating single-mode visible (619±640 nm) and infrared (3.16±3.77 lm) radiation. The single-mode (<400 MHz bandwidth) signal output energy of the OPO was limited to 10 lj due to residual idler re ections supporting a monolithic OPO within the PPLN crystal. A perylene red doped poly(methyl methacrylate) disc was used to amplify the signal wavelength up to 114 lj in a single pass without broadening the optical bandwidth. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: Yj Keywords: Optical parametric oscillators; Single-mode lasers; Solid-state dye ampli ers Quasi-phase matched (QPM) optical parametric oscillators (OPOs) based on periodically poled lithium niobate (PPLN) have recently been the subject of considerable research activity [1]. A signi cant bene t from employing QPM instead of birefringent phase matching is that any interaction within the transparency range of the base material may be phase matched using the highest nonlinear tensor coe cient [2]. Lithium niobate (LN), which o ers a high nonlinear coe cient of d 33 ˆ 27 pm/ V, is ferroelectric, so the periodic domain inversion * Corresponding author. Tel.: ; fax: address: pschlup@physics.otago.ac.nz (P. Schlup). required for QPM can readily be produced by an electric eld poling technique [1,2]. Thus, PPLN is a suitable candidate for OPO devices with the capability of providing laser-like radiation that is widely tunable in wavelength with potential applicability in areas, such as spectroscopy and environmental sensing [3]. However, LN exhibits a high coercive eld and a low optical damage threshold, which result in limited thickness PPLN crystals (typically <1 mm) that can be pumped only at the few-mj level. Furthermore, under nanosecond pumping, OPOs characteristically operate with optical bandwidths in excess of that required to perform some applications [3] and a form of line width control is needed. The work presented here addresses both the optical bandwidth and low output energy limitations /00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S (00)

2 226 P. Schlup et al. / Optics Communications 184 (2000) 225±230 Active as well as passive methods for narrowing the outputs of OPOs have been demonstrated in PPLN [4±9], but research has focused on devices pumped in the near-infrared (for instance by Nd:YAG lasers) to produce signal and idler outputs in the near and mid-ir, respectively. Pruneri et al. [10] have previously reported a then state-ofthe-art 200-lm thick PPLN crystal with 6.8 lm period in a near-degenerate OPO pumped by 12 lj, 532 nm pulses. More recently, Bader et al. [11] have demonstrated an optical parametric generator (OPG) based on PPLN, pumped by 90 lj pulses and producing signal pulses of up to 10.9 lj at a repetition rate of 1.1 khz. Maturation of electric eld poling technology has improved the crystal quality and poling depth achievable with shorter domain lengths, and poling periods of 6 10 lm required for visible-pumped OPOs are now becoming commercially available. In this work, the output from a nanosecond PPLN-OPO pumped in the visible region by the 532 nm second harmonic of a Nd:YAG laser is reduced to single longitudinal mode by the implementation of an intracavity di raction grating at grazing incidence. This line width control technique was rst demonstrated in dye lasers [12±14], but has since been used successfully in titanium-doped sapphire lasers [15] as well as OPOs based on potassium titanyl phosphate [16], barium borate [17], and more recently, infrared-pumped PPLN [8,9]. The high oscillation threshold of the grazing incidence cavity, caused by the high e ective loss of the di raction grating, and the observed parasitic processes of monolithic oscillation, limit the achievable signal and idler energies. We have used a perylene red dye doped polymer laser ampli er to address this limitation. Solid state dye gain media are well known to act as high gain, broadly tunable, compact and low-cost optical ampli ers. Their principal limitations are photo- and thermally induced molecular degradation which limit the operating lifetime. In this context, emerging dyes such as the perylene series [18±20] are attractive, as they o er e cient operation with high photostability. Perylene red, in particular, is one of the most photostable dyes reported [18]. Its gain is limited by excited state absorption, but its energy band structure is otherwise well suited to the present application with strong absorption at our 532 nm pump wavelength and a strong uorescence band matching the 619±640 nm tuning range of our PPLN OPO. Perylene red dye is nonpolar making it compatible with the poly(methyl methacrylate) (PMMA) host used in this work without the need for a solvent that can subsequently outgas and degrade the optical quality of the host. The simple and compact line-narrowed OPOampli er con guration makes the device, shown in Fig. 1, attractive for eld work. A frequencydoubled, injection-seeded, Q-switched Nd:YAG oscillator±ampli er laser (modi ed Continuum Powerlite PL7000) was used to pump the OPO. The laser emitted smooth and stable 7-ns pulses at a repetition rate of 10 Hz. The Fresnel re ection from the front face of a fused silica wedge was used to direct approximately 4% of the pump energy to the OPO. A half-wave plate and polariser combination allowed for smooth variable control of the pump energy at the OPO as well as ensuring the required polarisation (parallel to the z axis of the crystal) of the pump beam. The pump beam was focused by a 500-mm focal length achromatic lens with the PPLN crystal placed about 5 cm beyond the waist to ensure the pump beam lled the available crystal volume. The pump energy was Fig. 1. Experimental con guration: W, fused silica wedge; P, polariser; k=2, half-wave plate; HR, signal high re ector and PPLN, multiple track crystal. A 1: M perylene red doped PMMA solid-state dye disc was used to amplify the OPO signal output. Synchronisation of the pump and signal pulses at the dye was achieved by varying the steering mirror separation as indicated.

3 P. Schlup et al. / Optics Communications 184 (2000) 225± restricted to less than 0.5 mj to prevent bulk or coating damage to the crystal. The OPO cavity consisted of a plane dielectric mirror, a multiple-grating PPLN crystal, a metallic di raction grating at grazing incidence and a plane metallic tuning mirror. The pump beam entered the cavity through the dielectric mirror, which was coated for high re ectivity over 570±640 nm while maintaining 76% transmission at the 532 nm pump wavelength. The :5 mm 3 (length width thickness) PPLN crystal, manufactured by Deltronic Crystal Industries, had six adjacent poled regions with poling periods of 6.5, 10.0, 10.3, 10.5, 10.8 and 12.1 lm. The 6.5 and 12.1 lm poling periods could not be used for parametric oscillation with the pump wavelength and crystal temperatures used, so the results presented here are for the 10.0±10.8 lm poling periods only. These correspond to signal wavelengths between 619 and 640 nm with the associated idler wavelength ranging between 3.77 and 3.16 lm, respectively. The end faces of the crystal were polished and antire ection coated for the 532±650 and 1200±1300 nm wavelength ranges. The PPLN crystal was mounted in a temperature-controlled oven and held at 160 0:1 C to prevent photorefractive damage. Mounting of the crystal oven on a translation stage permitted accurate control of crystal placement as well as track selection. When aligning the OPO, the di raction grating was inserted into the cavity at grazing incidence. The rst di raction order was re ected back into the PPLN crystal by the tuning mirror, while the specular re ection (zeroth di raction order) was used as the OPO output. Two di raction gratings, with 1800 and 2400 grooves/mm (g/mm), were tried in the cavity. It was found that the 1800 g/mm grating resulted in better stability and higher e ciencies and that the increased dispersion of the 2400 g/mm grating was not necessary to achieve single-mode operation. The remaining work was performed using the 1800 g/mm di raction grating. The physical cavity length, measured to the centres of the di raction grating and turning mirror, was around 40 mm. The output of the OPO was collimated using a single 200-mm focal length lens, anti-re ection coated at visible wavelengths. For analysis, the signal beam was separated from the residual pump beam by a dielectric mirror coated for high re ectivity at 532 nm, followed by a Brewster-cut SF10 glass prism. The optical bandwidth of the OPO signal radiation was measured using a 5 GHz free spectral range glass Fabry±Perot interferometer with a resolution limit of around 250 MHz full width at half maximum (FWHM) intensity in the visible region. The interferometer fringes were imaged onto a CCD camera and interpreted using a computer program. Fig. 2 shows interferometer transmission traces with the OPO running at signal wavelengths of (a) nm, (b) nm, (c) nm and (d) nm. The traces indicate a single longitudinal mode signal output with an optical bandwidth of less than 400 MHz FWHM was obtained across the tuning range accessible by track selection. The oscillation threshold energy of the OPO was measured to be 230 lj per pulse across the tuning range used. It was found that single-mode operation could not be maintained with pump energies in excess of approximately 300 lj. Beyond this pumping level, the residual idler re ections o the crystal end faces were su ciently strong to support parametric oscillation on only the crystal, Fig. 2. Intensity pro les through the centre of the fringe pattern formed by a 5 GHz free spectral range Fabry±Perot interferometer, with the OPO operating at wavelengths of (a) nm, (b) nm, (c) nm and (d) nm. All traces demonstrate the OPO operating on a single mode with an optical bandwidth of <400 MHz FWHM.

4 228 P. Schlup et al. / Optics Communications 184 (2000) 225±230 thereby rendering the grazing-incidence di raction cavity ine ective. This was initially veri ed by measurements of injection seeding on adjacent modes of this monolithic OPO. A 635-nm laser diode (Hitachi HL6312G) was collimated and propagated collinearly with the pump beam through the monolithic OPO crystal. The diode operated on a single axial mode and could be continuously current tuned across a range of up to 50 GHz. Successful seeding was observed on adjacent modes separated by 3.8 GHz, which agrees well with the mode spacing predicted, using published Sellmeier equations [21], for idler resonance within the PPLN crystal. Final proof of idler resonance was o ered by tilting the free-running crystal in the vertical plane (orthogonal to crystal z axis). The misalignment of the idler-resonant cavity resulted in a de ection of the idler beam perpendicular to the crystal faces and a walk-o of the signal beam in the opposite direction, as required by phase matching. The problem of idler resonance is more acute in this system than in previously reported infrared-pumped OPOs where the gain is lower. Idler resonance could be overcome by more optimum anti-re ection coatings or by employing a crystal with wedged end faces. The latter was used in Ref. [11] to inhibit monolithic oscillation, where also injection seeding using a xed-frequency He±Ne laser was used to reduce the optical bandwidth to less than 400 MHz. For the present case, however, we elected to operate the OPO close to threshold and use the residual 532 nm energy to pump the ampli er, as discussed below. The maximum single-mode signal pulse energy of 10 lj was measured at a pump energy of 330 lj at a signal wavelength of nm, corresponding to a slope e ciency of 10% for the signal branch only. A peak signal slope e ciency of 15% was measured at a signal wavelength of nm, where 9.1 lj of signal radiation were obtained at a pump energy of 290 lj. The idler pulse energies were too low to be measured with the available power meters. Backconversion and nonlinear pump absorption e ects, which have been reported to limit the conversion e ciency of a 532-nm pumped OPG [11], were not observed under these operating conditions. The e ect of pump optical bandwidth was brie y investigated by turning o the injection seed on the pump laser. The resulting broadening of the pump optical bandwidth to several tens of GHz led to the OPO running on multiple modes. The temporal characteristics of the single-mode OPO output pulses were determined using a 1 ns rise time photodiode and displayed on a digital oscilloscope. The 3 ns pulses had a temporal stability of 1 ns, attributable to the timing jitter of the pump pulses, and an amplitude stability of 10% despite operating at only 1.3 times threshold. The single axial mode OPO signal radiation had a smooth spatial pro le and a beam quality parameter of M 2 ˆ 1:2. The OPO operated on a stable single axial mode over many pulses despite no active locking mechanisms. The implementation of an established active locking strategy should procure long-term stability of the OPO and enable single-mode scanning [14,16]. A simple single-pass ampli er of the signal wavelength was implemented using a 9.2-mm thick solid-state dye sample. The PMMA host was doped with perylene red at a concentration of 1: M. The dye was pumped by the same frequency-doubled Nd:YAG laser used to drive the OPO, with a separate half-wave plate and polariser combination allowing for independent smooth control of the pump energy at the dye. As no focusing or other control of the pump mode size at the dye sample was implemented, the spot size of approximately 1.5 mm was solely due to the propagation distance from the pump source. Spatial overlap between the OPO signal and dye pump beams was achieved by the use of a dielectric optic coated for high re ectivity over 570±640 nm and high transmission at 532 nm. Temporal overlap of the two pulses was accomplished by adjusting the relative position of steering mirrors as indicated in Fig. 1. The dye sample was mounted at BrewsterÕs angle to minimise re ection losses. Up to the maximum pumping levels of 20 mj reported here, the dye sample absorbed all of the 532-nm pump energy. Since the dye was not contained within a resonant cavity with optical feedback, the dye sample acted as a single-pass ampli er for the OPO signal

5 P. Schlup et al. / Optics Communications 184 (2000) 225± Fig. 3. Intensity pro le through the centre of the fringe pattern formed by a 5 GHz free spectral range interferometer of the ampli ed OPO signal wavelength of (a) nm and (b) nm. radiation. With the OPO signal beam blocked, no radiation was detected, con rming the sample did not lase. Fig. 3 shows representative traces of Fabry±Perot interferometer fringes generated by the ampli ed OPO signal with the OPO operating at (a) nm and (b) nm. The optical bandwidth of the ampli ed pulses is <400 MHz FWHM, comparable to that of OPO signal radiation without ampli cation. Fig. 4 shows the measured OPO signal as a function of ampli er pump energy for di erent Fig. 4. Ampli ed single-mode OPO signal pulse energy as a function of ampli er pump energy. The decreasing e ciency with increasing wavelength is attributed to the wavelength dependence of the dye gain. wavelengths. In all cases, the OPO was pumped at 300 lj to maintain single-mode operation; the dye was pumped with not more than 20 mj as a precaution to prevent physical damage to the dye sample. The maximum ampli ed single-mode signal pulse energy varied across the OPO tuning range with 114 lj at nm and 40 lj at nm. This corresponds to single pass gain of between 4 and 11 at 20 mj ampli er pump energy. The wavelength dependence of the gain is attributed to the spectral uorescence pro le of perylene red, which peaks at 605 nm with an FWHM range of 590±660 nm [18]. Since the radiation was ampli ed in a single pass, no ne spectral structure was introduced by the ampli cation process. This is particularly important for continuously tunable OPO operation. The temporal shape and duration of the ampli ed pulses were found to be una ected by the ampli er pump energy. However, the propensity of solid state dyes to thermal lensing, which resulted in asymmetric distortion because the dye sample was mounted at Brewster's angle, caused a deterioration of the spatial beam distribution especially at high dye pump powers. Beam quality measurements indicated M 2 2 when the dye is pumped by 7.5 mj, increasing to M 2 8 when the pump energy is increased to 20 mj. The resulting beams were heavily astigmatic. In summary, we have demonstrated a grazingincidence OPO capable of generating stepwise tunable, single-axial-mode (<400 MHz FWHM optical bandwidth) visible and infrared radiation. The single-mode signal output energy of the OPO was limited to 10 lj due to residual re ections supporting a monolithic OPO within the PPLN crystal. A perylene red doped PMMA solid state dye ampli er with single pass gain of 4± 11 was implemented to amplify the single-mode signal pulse to a maximum output energy of 114 lj with no measured broadening of the optical bandwidth. It is anticipated that other emerging QPM materials, such as periodically poled KTi- OPO 4, o ering the option of energy scaling due to increased sample thickness and damage threshold at the expense of a reduced nonlinear coe cient, may also be operated under similar schemes.

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