1 Introduction

Optical parametric oscillators (OPOs) have been established to be highly efficient sources of coherent tunable radiation. The attractive features of these laser sources include all-solid-state compact design and the possibility of very broad wavelength tuning range using just a single nonlinear crystal. However, pulse repetition rate of OPO is still limited compared with widely used dye lasers. Currently, type-I phase matched β-Ba2BO4 (BBO) crystal-based OPO, pumped by the third harmonic of a Q-switched Nd:YAG laser with output wavelength tunable from 412 to 2.7 µm [1, 2], serves as the most versatile practical OPO for wide ranging spectroscopic applications [3]. Despite its superior characteristics as a widely tunable coherent source, visible-NIR pulsed OPOs have not yet been in use where the applications require high pulse repetition frequency (PRF) operation such as in laser photoionization spectroscopy for trace analysis, metrology, and LIDAR. Visible-NIR OPOs [4] exhibit a high-pump (UV) pulse energy threshold and stringent demands on spatial and temporal quality of the pump beam, which is the main limitation towards development of such high repetition rate and high average power OPOs. For example, a 355 nm pumped BBO crystal-based OPO with a crystal length of about 15 mm would require a few tens of mJ of pump energy [5], to work comfortably above threshold when operated as a broadband OPO. Reducing the threshold pulse energy by tight focusing the beam is counterproductive, due to the beam walk-off effects caused by the double refraction. The non-critical phase-matching condition, in which walk-off caused by the double refraction is zero, allows focusing the pump beam tightly and reduces the pulse energy requirement of the pump laser. However, in case of the BBO crystal, achieving non-critical phase matching by varying temperature is not possible because of its low temperature coefficient of birefringence.

In our earlier work on OPO, a substantial reduction in the pump threshold energy was achieved, down to less than 1 mJ/pulse at 10 Hz PRF [5], which would facilitate the development of high PRF visible-IR OPOs. We utilized gainfully the advantage that the cylindrical focusing geometry increases the interaction length of the pump and the signal within the crystal in the walk-off sensitive plane, while high field intensity is simultaneously maintained as the beam is focused in the insensitive plane. Thus, we have adopted the similar pump focusing geometry in our present OPO setup for demonstrating high repetition rate operation. In this paper, we report high repetition rate (5 kHz), nanosecond type I BBO OPO, which can be tuned from 490 to 630 nm. Herein, we describe the experimental setup and signal output performance of the high repetition rate OPO.

The free running type-I phase-matched BBO OPOs exhibit broad spectral linewidth. The spectral linewidth of type I BBO OPOs varies from sub nanometer to few nanometers as the wavelength is tuned toward degeneracy. Alternatively, type-II phase matching exhibits much narrower linewidth throughout the entire tuning range, typically 0.1–0.5 nm near degeneracy [6,7,8], when pumped by the third harmonic (355 nm) of the pump laser. However, divergence broadening in type-II phase matching is more predominant and it is much larger than the inherent spectral linewidth arising due to parametric process. As a result, the linewidth of type-II phase matching OPO is mainly determined by divergence broadening. Anstett et al. [8] have developed a technique in which they have double passed the pump beam using a pump beam reflector for reducing the divergence broadening. With this technique, they were able to reduce the linewidth of type-II BBO OPO to a factor of 20, less than 0.1 nm. We have adopted the similar technique of double passing the pump beam in type-II BBO OPO for demonstrating high repetition as well as rate narrow linewidth operation. Since, the pump beam is back reflected, we have restricted the maximum repetition rate of pump laser to 2 kHz to avoid any damage of optical components and crystal.

At high repetition rate (5 kHz) of pump laser beam and with pulse energies of 3–4 mJ, the residual absorption of optical powers within the BBO crystal cannot be neglected. This leads to inhomogeneous heating of the nonlinear optical crystal that results in spatial temperature gradient and refractive index changes. The spatial inhomogeneous refractive index changes leads to spatially varying phase-matching conditions, limiting the attainable average power, bandwidth and beam quality [9, 10]. We have studied the effect of pump beam absorption on the performance of the type-II BBO-based OPO. The performance of the OPO has been evaluated by studying the variation of output signal power with input pump power at different repetition rates. The temperature gradient arising due to absorption of the pump beam has been measured by imaging the crystal using an infrared camera to ensure that the temperature gradient should remain much lower than the acceptable temperature limit. It may be noted that Li et al. [11] demonstrated high PRF (10 kHz) operation of broad band, OPO based on a long LiB3O5 crystal, pumped by second harmonic (532 nm) of a diode-pumped solid state laser (DPSSL), which gave signal output up to 9.4 W at 900 nm for pump power of 18 W inside the LBO crystal. The present report is, to the best of our knowledge, the highest repetition rate operation of nanosecond visible-NIR pulsed OPO, pumped by the third harmonic of a DPSSL.

2 Experimental setup and results

2.1 High repetition rate, type-1 BBO crystal OPO

The schematic of experimental setup of high repetition rate OPO is shown in Fig. 1. The OPO consists of a type I phase-matched BBO crystal inside a cavity formed by flat–flat mirrors. The input cavity mirror (M3) possesses high reflectivity > 99% in the spectral region 490–630 nm and the output coupler (M4) is coated for 70% reflectivity in the same spectral region [5, 8], which is estimated to be the optimum reflectivity. Both mirrors have high transmission at pump and idler wavelengths to prevent doubly resonant operation. In this OPO setup, we have used three available different gain lengths of BBO crystals (make: Castech, China) of dimensions 6 × 8 × 12, 6 × 8 × 15 and 6 × 8 × 18 mm3 which are cut (θ = 30°, φ = 0) for type-I phase matching and the corresponding OPO cavity lengths for the above BBO crystals are 22, 25 and 28 mm, respectively. The crystal faces are coated with high damage threshold protective coating film at 355 nm.

Fig. 1
figure 1

Schematic of the experimental setup of type-I phase matched BBO crystal-based OPO. M1, M2: pump beam routing mirror, M3, M4: OPO cavity mirrors, M5: pump beam reflector, M6, M7: signal beam reflectors

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The pump source is the third harmonic (355 nm) of a diode-pumped Nd:YAG laser (make: M/S Edgewave GmbH, Germany). The laser delivers a smooth temporal profile with 10 ns full width half maximum (FWHM) at 5 kHz PRF with pulse energy of ~ 3.5 mJ at 355 nm and the energy stability is about less than 2%. The repetition rate of the DPSSL system can be varied from 1 to 5 kHz. The DPSSL provides a beam of size 5 mm × 5 mm with a far-field beam divergence of less than 0.5 mrad.

The maximum output power of the pump laser at 355 nm is 16 W. The output beam from the pump laser has been routed through the variable attenuator, comprising of a half-wave plate and a polarizer at 355 nm, which can be allowed to vary the output power of the pump laser. The attenuated pump beam is allowed to pass though the cylindrical focusing geometry, which compresses the pump beam in the horizontal plane by a cylindrical telescope comprising of lenses L1 and L2 as shown in Fig. 1 and focused using a 500 mm focal length spherical lens in the non walk-off plane of the crystal, discussed in Ref. [5]. The focal lengths of L1 and L2 are 100 and 50 mm, respectively. The focused incident pump beam size at the crystal surface is estimated to be 5 mm (horizontal walk off plane) and 0.4 mm (vertical).

As shown in the Fig. 1, the residual pump after the exit mirror is routed to the beam dump using the dichroic mirror M5. The mirror M5 has high reflectivity for pump wavelength and high transmission for signal wavelengths. The signal beam is then routed to power meter and mono-chromator using the mirrors M6 and M7. Both the mirrors have high reflectivity at the signal wavelengths and high transmission for pump and idler wavelengths so as to block the pump and the idler from reaching the power and wavelength measurement systems.

With the available set of mirrors, the type I BBO crystal OPO signal could be continuously tuned from 490 to 630 nm, corresponding to calculated idler wavelengths from 1288.5 to 813.3 nm. The tuning range is limited by reflectivity profile of the present mirrors. The variation of signal output power, measured at a typical signal wavelength 492 nm as a function of the pump power at 5 kHz repetition rate, for different BBO crystal sizes are shown in Fig. 2. The output signal power increases with the input pump power in all three cases. We have achieved a maximum output power of 3.23 W with an input pump power of 11.5 W, corresponding to a conversion efficiency of 28%. It may be noted that the single pass gain of the OPO signal possesses quadratic dependence on the interaction length. Hence, the output power of the OPO enhances with the increase in the crystal length. However, the beam walk-off effect dominates with increasing length of the crystal to more than an optimum value which degrades the performance of the OPO. Consequently, the slope efficiency of the OPO was observed to increase from 26.8 to 33.4%, and the threshold pump power decreases from 5.6 to 2.55 W when the crystal length was changed from 12 to 15 mm. However, the increase in the slope efficiency (0.8%) and the decrease in threshold (0.05 W) were nominal, when the crystal length was increased from 15 to 18 mm, because of beam walk-off effect.

Fig. 2
figure 2

Performance comparison of signal output power of type-I BBO OPO for different crystal sizes at 5 kHz repetition rate at a signal wavelength 492 nm. The symbols represent experimentally observed results and the solid line represents the best linear fit

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Thus, 15 mm is fixed as optimum length of the crystal for the present OPO system for achieving reasonably high slope efficiency and low threshold operation using cylindrical focusing geometry. Figure 3 shows the variation of measured signal output power when the OPO signal output is tuned from 490 to 630 nm at a fixed pump power of 10 W. The reduction in the efficiency of OPO system during tuning output signal from 490 to 630 nm is due to decrease in the effective nonlinear coefficient of the BBO crystal with the increase in signal wavelength and further due to higher reflection losses at the crystal surface.

Fig. 3
figure 3

Variation of OPO signal output power tuned from 490 to 630 nm at a fixed pump power of 10 W. The symbols represent the experimental data and the solid line show the trend in results

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The power stability of the signal output of type-I BBO OPO system with 15 mm long crystal is studied by monitoring the output power at 492 nm when operating the pump laser at a repetition rate of 5 kHz. The pulse width of the generated signal output is 6 ns (FWHM). Figure 4 shows the long-term stability of the OPO signal output power monitored for duration of 30 min using a power meter (Ophir, Israel). During this operation, the OPO is operated with an average power of 2.6 W at a stability of ± 1.9% for pump power at 10 W.

Fig. 4
figure 4

Long-term stability of the type-I BBO OPO signal output power monitored for the duration 30 min

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The spectral linewidth of the type-I BBO OPO signal output is measured at different wavelengths of the tuning range from 490 to 590 nm using HR 2000 plus spectrometer (Ocean Optics). The spectrometer can measure the wavelength range from 490 to 590 nm with an optical resolution of ~ 0.035 nm. The measurement of the spectral linewidth beyond 590 nm is limited by the spectrometer. The variation of spectral linewidth of OPO output with signal wavelength is illustrated in Fig. 5. The spectral linewidth of the OPO increases from 0.2 to 0.5 nm when tuned from 490 to 590 nm.

Fig. 5
figure 5

Spectral linewidth of type-I BBO OPO as a function of signal wavelength

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2.2 High repetition rate, type-II BBO crystal OPO

The type-II OPOs exhibit high pulse threshold energy and low conversion efficiency because of lower nonlinear coefficient values and larger tuning angles for covering the entire tuning range compared to type-I [7]. Thus, it is customary to use long and large aperture size crystals for efficient operation. In this OPO setup, we have used a larger size BBO crystal of dimension 6 × 12 × 20 mm3, which is cut (θ = 37°, φ = 30) for type-II phase matching and the crystal faces are coated with high damage threshold protective coating film at 355 nm. The type-II BBO OPO setup is similar to the type-I BBO OPO setup as shown in Fig. 1, except the type-I BBO crystal is replaced with a large size type-II BBO crystal and the cavity length is increased to 30 mm.

The signal output from type II BBO crystal OPO can be continuously tuned from 490 to 630 nm with an angular variation of 10°. The performance of the OPO signal output was studied at the peak wavelength 512 nm, for different repetition rates of the pump laser. The variation of signal output power measured at pump laser operating at different repetition rates (1–4 kHz) as a function of input pump power is illustrated in Fig. 6. The output signal power increases with the input pump power. However, the performance of the OPO starts to degrade if the pump beam is operated beyond 3 kHz PRF. The threshold pulse energy of our type-II BBO OPO system is 1 mJ and we have obtained a slope efficiency of about 23% when operated with pump laser at 3 kHz. The output power of the OPO is observed to be very unstable and fluctuating with time, if we operate the pump laser beyond 3 kHz, and the pulse energy is more than 2 mJ. The degradation of OPO power is attributed mainly to the residual absorption of pump power resulting into inhomogeneous heating of the crystal, which is discussed in Sect. 2.4.

Fig. 6
figure 6

Signal output performance of the type-II BBO OPO at different repetition rates of the pump laser. The symbols represent the experimental data and the solid line show the trend in results

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The spectral linewidth of the type-II BBO OPO signal output was measured at different wavelengths of the tuning range from 490 to 590 nm at a PRF of 2 kHz. The variation of linewidth with wavelength is illustrated in Fig. 7. The spectral linewidth of this free running OPO varies between 0.53 and 0.65 nm across its tuning range.

Fig. 7
figure 7

Spectral linewidth of type-II BBO OPO as a function of signal wavelength

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2.3 High repetition rate, narrowband OPO

The spectral linewidth of the pulsed OPOs signal output depends on pump beam spectral linewidth, OPO cavity length and divergence of the generated signal output [12, 13]. The type-II phase matching OPOs exhibit much narrower linewidth in comparison to type-I phase matching OPOs [7, 8]. However, the influence of divergence of the resonant signal wavelength on the spectral linewidth in type-II phase matching is more than that of type-I phase matching due to asymmetric change of the resonant OPO wavelength with non-collinear angle [8]. Hence, in case of type-II phase matching OPOs, divergence broadening mainly determines the spectral linewdith of the signal output.

In this case, the increase in the spectral linewidth of type-II BBO OPO when compared to type-I BBO OPO is mainly due to divergence broadening. To reduce the divergence of the generated signal output, we have placed a pump beam reflector inside the type-II BBO OPO cavity in between the crystal and output coupler for double passing the pump beam inside the crystal as shown in Fig. 8. The pump beam reflector is a flat mirror, dielectric coated for high reflectivity (> 99%) at 355 nm for normal incidence and high transmission for signal and idler wavelengths. The advantage of double passing the pump beam configuration is that it will not only increase the gain of the signal because of the amplification of signal wave in both forward and backward directions and therefore decrease the oscillation threshold [14], but also decreases the divergence of the resonant signal wave as discussed in Ref. [8]. Reduction of the divergence of the resonant signal wave narrows the spectral linewidth to its inherent spectral linewidth arising mainly due to parametric process of the OPO.

Fig. 8
figure 8

Schematic of the experimental setup of narrowband OPO. M1, M2: pump beam routing mirror, M3, M4: OPO cavity mirrors, M5: pump beam reflector, M6, M7: signal beam reflectors, R1: pump beam reflector

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The spectral linewidth of the signal output from type-II BBO OPO with double pass configuration of the pump beam is measured at different wavelengths of the tuning range from 490 to 590 nm at a repetition rate of 2 kHz. A comparison of spectral linewidth of the signal wavelengths obtained from all three OPO setups (type-I BBO OPO, type-II BBO OPO and type-II BBO OPO with pump double pass configuration) are shown in Fig. 9. By employing pump double pass configuration for type-II BBO OPO, the spectral linewidth of the signal output is reduced by a factor of 4 when compared to free running type-II BBO OPO.

Fig. 9
figure 9

Comparison of spectral linewidth of type-I, type-II BBO and type-II BBO OPO using pump double pass configuration

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The performance of the type-II BBO OPO with pump double pass configuration is compared with the free running type-II BBO OPO at a signal wavelength of 512 nm, when the pump laser is operated at a repetition rate of 2 kHz. The variation of signal output power measured with input pump power for both the OPO setups are shown in Fig. 10. The performance of the OPO is found to improve in pump double pass configuration when compared to free running operation. In pump double pass configuration, the OPO operates at a maximum output power of 800 mW at 2 kHz repetition rate with a conversion efficiency of 18.2% at 512 nm and the threshold power is around 1.5 W. Whereas in free running operation, the conversion efficiency drops to 15% and the threshold power is increased to 2 W.

Fig. 10
figure 10

Performance comparison of type-II BBO OPO with type-II BBO OPO using pump double pass configuration. The symbols represent experimentally observed results and the solid line represents the best linear fit

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2.4 Thermal loading due to high average power operation

The power and wavelength stability of the high average power optical parametric oscillators are affected by the thermal effects arising from the optical absorption in nonlinear crystals. In our setup, type-II BBO OPO, the performance of the OPO is shown to degrade beyond 3 kHz repetition rate operation. The deterioration of the performance is attributed to heating of the crystal due to absorption of pump laser beam. The temperature rise in the crystal induces the change in phase-matching conditions which causes the reduction in the gain of the signal output. The allowed maximum temperature change for type-II BBO crystal of 20 mm length is 8.45 K and for type-I BBO crystal of length 15 mm is 11.16 K [15]. The crystal peak temperatures are measured using thermocamera (ThemaCAM Researcher, FLIR systems). From temperature measurements, it is found that the pump beam absorption is the main source of heating and the absorption due to signal and idler is negligible. Therefore, crystal temperature measurements are carried out in presence of pump beam only. All the BBO crystals used in our OPO setups are wrapped with a 100 µm thick indium foil to ensure good thermal contact over the whole crystal length and mounted in copper holder for efficient heat removal. The thermal conductivity and thermomechanical properties of the BBO crystals are reported in the Refs. [16, 17].

Figure 11 illustrates the measured peak temperatures at the crystal input surface of type-I and type-II BBO crystals versus the repetition rate of the pump laser (room temperature ~ 26 °C) with constant pump pulse energy 2.25 mJ. At high average power above 3 kHz repetition rate, the temperature gradient of type-II BBO crystal is found to be greater than its acceptable temperature bandwidth. Consequently, the type-II BBO OPO performance is observed to degrade. From our temperature measurements, it is apparent that the type-II BBO crystals, exhibit much slower heat transfer rate because of their bigger size and volume, and hence the temperature gradient in type-II BBO crystals is higher compared to type-I BBO crystals. Thus, type-I BBO crystals with dimensions similar to our optimized crystal size are suitable for achieving high average power operation with repetition rate of 5 kHz at a pulse energy of around 2 mJ. Furthermore, for high average power narrowband operation, instead of using longer size type-II BBO crystal, the possibility of using highly efficient type-I BBO crystals integrated with narrowband techniques such as injection seeding [18], and grazing incidence OPO (GIOPO) [19] can be exploited.

Fig. 11
figure 11

Comparison of measured peak temperatures of type-I and type-II BBO crystals at different repetition rates of pump laser

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Since the temperature gradients are approaching the temperature tolerance limits, they may induce the stress inside the crystal which hinders the long-term operation of high repetition rate OPOs [20]. The temperature gradients must be minimized for damage free operation of these crystals. Heat management schemes such as cooling the side surfaces of the crystals for dissipating the heat more effectively, minimizing the heat accessions such as coatings of the crystals and by recent approach of sandwiching the crystal front and rear surfaces with the transparent and high thermal conductivity material like sapphire plate [21, 22] which has 40 times larger thermal conductivity than BBO crystals, are proposed to implement with improved designs of reduced thermal effects for future development of high average power narrowband visible OPO’s.

3 Conclusion

In conclusion, we have developed a broad band, high repetition rate (5 kHz), type-I BBO OPO, which can be continuously tuned from 490 to 630 nm. We compared the performance of the OPO with three different size type-I crystals of lengths 12, 15 and 18 mm, generating a maximum output of 3.23 W at an input power of 11.5 W with a conversion efficiency of ~ 28% for 15 mm crystal. The long-term stability of the OPO was tested by operating the OPO at its peak wavelength at a repetition rate of 5 kHz for about 30 min. During the continuous operation, the OPO operates with an average power of 2.6 W with a conversion efficiency of 26% with a power stability of 1.9%. We have studied the performance of type-II BBO OPO with larger size crystal of length 20 mm at different repetition rates of the pump laser. The performance of this OPO starts to degrade when the pump laser is operated beyond 3 kHz with pulse energy above 2 mJ, owing to the thermal effects arising due to the absorption of pump beam. For operating the OPO with narrowband output, we have employed pump beam reflector for double passing the pump beam in type-II BBO OPO, and achieved narrowband (0.15–0.19 nm) output at a repetition rate of 2 kHz in the wavelength range 490–590 nm. To explain the degraded performance of the type-II BBO OPO at high repetition rates, we have measured the increase in the temperatures of the BBO crystals with repetition rate of the pump lasers and discussed the influence of thermal effect on the performance of high average power OPOs.