1. Introduction

Synchronously-pumped optical parametric oscillators (OPOs) provide one of the most versatile platforms for the generation of tunable visible and infrared pulses [1–4] at MHz and GHz repetition rates [5–7], and have found widespread use in multi-photon imaging [8,9], spectroscopy [10,11] and metrology [12]. A typical synchronously-pumped OPO cavity constructed in a four-mirror linear or bow-tie ring configuration requires dielectric mirror coatings that provide high reflectivity over the signal or idler range with a high transmission window at the pump wavelength. An antireflection coating is also often applied to the rear surface of at least one mirror to ensure efficient transmission of the pump beam into the cavity. These high-reflectivity coatings may be designed with tailored group delay dispersion profiles to produce smooth cavity length tuning and to drive the formation of desired pulse shapes, often at the expense of bandwidth and at increased cost. Mirrorless [13] and dielectric-free [14] OPO cavity designs have been realised, however the former requires few- or sub-micron domain periods that are demanding to engineer, while the latter uses a complex pump-coupling arrangement that cannot be easily translated to different wavelength regions.

An alternative resonator design exploits total internal reflection in a solid prism or hollow retroreflector. A pair of Brewster angle prism retroreflectors [15] have been employed in high-finesse optical resonators for enhanced spectroscopy [16,17]. Cavity coupling is achieved by offsetting one prism away from Brewster’s angle, with the weak surface reflection from incident radiation forming the resonant beam. To maintain a stable cavity one prism surface is manufactured with a large radius of curvature, and all surfaces are super-polished to sub-nm RMS surface roughness to ensure high finesse.

In this paper we report for the first time a synchronously-pumped optical parametric oscillator cavity which employs a pair of Brewster’s angle Pellin-Broca prisms formed of all-planar surfaces, with intracavity lenses producing a stable Gaussian mode. Pumped by a high-repetition rate Ti:sapphire laser and using periodically-poled lithium niobate (PPLN) as the nonlinear medium, OPO input coupling is achieved with a longpass filter. From this simple cavity we measure chirped signal pulses tuneable over $1100- 1350$ nm with up to 95 mW average powers, and demonstrate external compression below $150$-fs using a double-pass prism pair. The cavity is compact, simple to construct and is highly stable, making it an attractive low-cost OPO platform for multi-photon microscopy applications.

2. Brewster angle prism retroreflector OPO

Here we review the basic properties of a Brewster angle prism retroreflector before detailing its implementation in a synchronously pumped OPO.

2.1 Prism design

A top-down schematic of a Brewster angle prism retroreflector is shown in Fig. 1. The prism is composed of an isotropic material of refractive index $n$, with side lengths $L_1$ and $L_2$ and acute angle $\alpha$. A ray entering the prism at Brewster’s angle $\theta _B$ at position R$_0$, located a distance $z$ from apex $A$, will undergo two total internal reflections at positions R$_1$ and R$_2$. These reflections have incident angles near $45^\circ$, such that $\beta + \gamma = 90^\circ$. The ray exits the prism at position R$_3$ and propagates parallel to the incoming beam path. The internal path traversed by the ray is given by R$_0$R$_1$ + R$_1$R$_2$ + R$_2$R$_3$, which can be found geometrically using the following expressions:

 

Fig. 1. Left panel: Diagram of a Brewster angle prism retroreflector - see text for details. Right panel: Change in optical path length traversed in prism as it is rotated around Brewster’s angle.

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2.2 OPO cavity

A schematic of the OPO is shown in Fig. 2. The OPO was pumped by a 333-MHz Ti:sapphire laser (Gigajet, Laser Quantum) which produced 1.25 W of average power and 30-fs pulses centered at 806 nm with a corresponding 0.9-m cavity length. The pump beam was coupled into the OPO using a 950 nm longpass filter (Edmund Optics 87-039) with >99% reflectance at the pump wavelength band and <2% reflectance over the range of $1100- 1600$ nm. A 20-mm BK7 plano-convex lens with a broadband anti-reflection coating across $750- 1550$ nm (Edmund Optics 38-409) focused the pump beam into a 1-mm-long PPLN crystal (HCP FOPMIR-FA) to produce a 15-$\mu$m beam waist. The crystal contained a fan-out grating structure which could be accessed through vertical translation, ranging in period from $\Lambda = 20.50~\mu$m to $\Lambda = 23.50~\mu$m, and the faces were anti-reflection coated for the pump ($R < 0.5\%$), signal ($R < 0.5\%$) and idler ($R < 5\%$) wavelengths. The crystal temperature was raised to 90 $^\circ$C to reduce photorefractive effects induced by parasitic frequency doubling.

 

Fig. 2. (a) Schematic of the Brewster angle prism retroreflector OPO. LPF - longpass filter; OC - sapphire output coupler; PPLN - periodically poled lithium niobate crystal. (b) Signal beam radius within the OPO cavity. Small deviations occur at the prism locations near 200 mm and 650 mm. Inset, signal waist in the 1-mm PPLN crystal, indicated in red.

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A second 20-mm lens collimated both the pump and signal beams, which passed through a pair of BK7 Pellin-Broca prisms (Thorlabs ADB-10). The prisms had a vertical aperture of 10 mm and an acute angle $\alpha$ of 78$^\circ$26’. When the signal beam entered the prism at Brewster’s angle the internal path was 44.7 mm, calculated using Eqs. (1) - (3). The first prism was mounted on a translation stage angled parallel to the incoming beam to provide cavity length control, while the second prism was mounted on a stage parallel to the Brewster input face to enable adjustment of the horizontal beam separation.

After being retroreflected by both prisms the depleted pump beam exited the OPO cavity by reflection from the inner surface of the longpass filter, while the resonant signal beam was refocused into the PPLN crystal. Signal spectra were obtained by inserting a 2-mm thick wedged sapphire plate at near-normal incidence before the first prism. A Fresnel reflection from the front surface provided $7.5\%$ output coupling, while the rear side of the plate was anti-reflection coated for the signal wavelength ($R < 1\%$). Idler wavelengths generated in the crystal experience a cumulative $\sim 40\%$ reflection and absorption loss from the BK7 collimating lens. The remaining power is absorbed in the first BK7 prism and does not resonate in the cavity. No evidence of absorption-induced thermal effects was observed. Insufficient idler power was reflected from the sapphire plate to enable spectral characterization.

3. Results and discussion

The calculated phase-matching bandwidth of the 1-mm PPLN crystal is shown in Fig. 3. A grating period of $\Lambda = 21.3~\mu$m will support signal wavelengths from $1100- 1600$ nm and beyond into degeneracy, with this bandwidth phase-matched within the full-width half-maximum (FWHM) of the pump spectrum. The net reflectivity of the cavity, shown in the upper panel of Fig. 3, is calculated from the AR coatings on the longpass input coupler, lenses, crystal, and output coupler. Careful selection of cavity optics produced a net roundtrip loss of $10\%$ across the phase-matched wavelength range, providing a threshold of 525 mW.

 

Fig. 3. Main panel: calculated phase-matching of 1-mm PPLN with $\Lambda = 21.3~\mu$m. Dotted lines indicate the phase-matched wavelengths that lie within the FWHM of the pump spectrum (right panel). Signal wavelengths from $1100-1600$ nm can be generated under the calculated net reflectivity curve of the OPO (upper panel).

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Typical signal spectra recorded when adjusting the cavity length in 20-$\mu$m increments are shown in Fig. 4(a), together with average powers for each spectrum. The center wavelength was tuneable from $1100- 1350$ nm as the OPO length was continually adjusted over 140 $\mu$m, with no realignment required after tuning. The cavity delay tuning range is larger than a comparable OPO operating in a low-dispersion regime [18]. Maximum average powers of 95 mW were recorded for 1.16 W of incident pump power, limited by the output coupler and by sub-optimal focusing conditions. The 20-mm lenses produced a focusing parameter of $\xi \approx 0.34$ [19], well below the optimal $\xi = 1$ for parametric interactions. Tighter focusing ($f = 12.7$ mm) was attempted however it led to damage of the PPLN crystal. No evidence for aberrations introduced by the lenses was observed. Figure 4(b) shows the pump depletion for each of the signal spectra displayed in Fig. 4(a), with an average depletion of $30\%$. The OPO power and spectra remained stable over a long timescale without enclosure or cavity length locking, as shown in Fig. 5. The RMS noise was $0.34\%$, measured over 1 hour. The signal beam profile (Fig. 5, inset) showed no evidence of aberrations or distortion.

 

Fig. 4. (a) Signal spectra and (right axis) measured average power. Spectra were acquired at 20 $\mu$m cavity length increments. (b) Corresponding depleted (solid colored lines) and undepleted (dotted black lines) pump spectra.

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Fig. 5. Stability of the signal wavelength (top) and output power (bottom), sampled at 2-second intervals. Inset: the signal beam profile.

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Instantaneous signal bandwidths of up to 15 nm could be achieved (measured at the FWHM) with broad pedestals covering up to 50 nm. Dual-band signal operation was observed at several tuning positions, with a weak secondary signal pulse present in the $1400- 1600$ nm region. Maximum powers of 10 mW were recorded when isolating this portion of the spectrum, preventing pulse characterization. Detuning of the cavity length did not produce single band operation beyond 1350 nm. This dual-band behavior is a result of the broadband cavity reflectivity and an inflection in the cavity round-trip group delay (GD) at 1338 nm, where the net group delay dispersion (GDD) passes through zero, as shown in Fig. 6. Similar dispersion-driven behavior has previously been observed in synchronous OPOs [20], but can also be attributed to appropriate group-velocity mismatching between the dual signal wavelengths and the pump laser [21].

 

Fig. 6. (a) Net cavity GD, offset relative to the minimum value. (b) Net cavity GDD.

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Pulse-widths in excess of 350-fs were measured with an intensity autocorrelator (A.P.E. pulseCheck-50), revealing a heavy degree of chirp due to the high net cavity dispersion. External compression was carried out using a double-passed pair of SF11 prisms with a 20-mm aperture and an average tip separation of 1 m. The pulses were characterised with a home-built second-harmonic frequency-resolved optical gating (SHG-FROG) apparatus, resulting in sub-150-fs sech$^2$ pulses (Fig. 7). Residual higher-order dispersion prevented compression to the transform limit of sub-70-fs.

 

Fig. 7. Top row: Experimental SHG-FROG traces. Second row: Retrieved FROG traces. Third row: Retrieved compressed pulse profiles. Bottom row: Experimental (blue) and retrieved (grey dashed) spectra.

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4. Conclusions

We have demonstrated a novel OPO cavity based on Brewster-angled prism retroreflectors. By exploiting broadband AR-coated optics, the OPO produces signal wavelengths across $1100- 1350$ nm with up to 95 mW average powers, along with weak dual-band operation across $1400- 1600$ nm. Large intracavity dispersion produces highly chirped pulses, but this may lend itself to chirped pumping when using a longer crystal [22]. In future work we will investigate the impact of the dispersive prisms on pulse formation using a full-field nonlinear envelope model [23].

This proof-of-principle cavity will be further explored by sourcing alternative Fresnel output couplers. Replacing the BK7 prisms and lens with less dispersive substrates such as CaF$_2$ should provide smoother tuning and will also allow for efficient idler extraction and characterization. The insertion of a second focusing section containing an appropriate material with high nonlinear refractive index could enable SPM-driven spectral broadening and the generation of shorter pulses [24]. Finally, selecting prisms with differing substrates that have complementary dispersion curves would allow the broadband AR coating range of the remaining optics to be fully exploited.