1. Introduction

Raman-shifting Nd lasers in crystalline media form an important class of 550–650nm radiation where very few other practical sources of powerful laser radiation exist. Such sources are all-solid-state, efficient and of interest for applications in remote sensing, visual display, dermatology and other areas of medicine.

By far the highest average Raman laser powers in the yellow or orange to date have been obtained using intra-cavity configurations. Using LiIO₃, 1.2W of 578nm output at 10kHz pulse rate was generated using an intracavity Raman-shifted Nd:YAG laser frequency doubled in LBO [1]. External Raman resonators, however, have an important attraction in that they are a versatile and simple add-on to an unaltered pump laser. If we consider Nd lasers as a fundamental source of laser photons, there are two major routes to yellow/orange generation. In one scheme, the fundamental IR laser output of the pump laser is Raman shifted and the resulting output converted to the visible by second harmonic generation (or by sum frequency generation with the residual fundamental). Using this scheme, efficient first Stokes output at 1158 nm was generated in the Raman material Ba(NO₃)₂ pumped by a Nd:YAG laser operating at 4kHz pulse rate [2], however, thermal birefringence induced in the Ba(NO₃)₂ resulted in elliptically-polarised first Stokes output which limited the conversion efficiency to the yellow second-harmonic. In a second scheme, which we report on here, the IR pump laser is frequency-doubled prior to Raman downshifting. As a result, thermal effects introduced into the optical system by the inelastic Raman process occur downstream of the second harmonic generation step and thus cannot reduce its conversion efficiency. The scheme also takes advantage of the higher Raman gain in the visible (Raman gain scales inversely with wavelength for nanosecond pump lasers [3]) thereby reducing the Raman laser threshold. Moreover, the spacing of accessible Stokes orders is twice as frequent compared to that obtained when the second-harmonic step is performed last, and visible output may be obtained in single or multiple lines operation depending upon the choice of output coupler.

External solid-state Raman resonators operating in the visible have been studied for a number of high pulse energy flashlamp-pumped laser systems (see Refs. [4,5] and references contained therein). For example, He and Chyba obtained yellow-orange output with up to ~45% conversion efficiency using a BaNO₃ Raman resonator pumped by 532 nm pulses of energy up to 30mJ at 10–30 Hz repetition rate [6]. However, to our knowledge Raman conversion to the yellow-orange by external cavity resonators has not as yet been investigated in detail for multikilohertz, low pulse energy pump sources typical of diode-pumped Q-switched Nd lasers.

In this paper, we report efficient yellow, orange and red output at multi-kilohertz pulse rates using an external Raman oscillator pumped by a compact 1W 532nm source with pulse energies <250µJ. Specifically, we used an air-cooled 532nm laser module and a Raman oscillator based on KGd(WO₄)₂. KGd(WO₄)₂ is an increasingly popular Raman material that features a higher thermal conductivity, lower dn/dT and a higher damage threshold compared to other more popular Raman materials such as Ba(NO₃)₂. Moreover, KGd(WO₄)₂ has a monoclinic lattice structure which causes the optical (and thermophysical) properties to be anisotropic [7], and the Raman gain to be polarization-dependent on two strong Stokes lines at 768cm^-1 and 902cm^-1 (corresponding to the pump polarization oriented along the N_m and N_g crystal axes). Output is therefore possible on two wavelengths for each Stokes order depending on the crystal rotation about the laser axis, giving rise to a greater diversity of output wavelengths.

2. Experiment

The experimental arrangement is shown in Fig. 1. The pump laser was a diode-pumped, coplanar folded Nd:YAG slab laser (previously described in ref. [8]) repetitively Q-switched at 5kHz (pulse duration 10ns) and extra-cavity frequency doubled. The output polarization was oriented to the vertical using a half-waveplate and focused into the KGd(WO₄)₂ crystal using a 100mm focal length lens. The KGd(WO₄)₂ crystal (supplied by EKSMA), cut for laser propagation along the b-axis, had dimensions 5 mm x 5 mm x 50mm and was AR coated for 532–600 nm. The Raman resonator was a 3 mirror cavity folded by a plane dichroic input mirror placed at ~60⁰ to the incident s-polarized pump. We chose this angle for maximum reflection for the Stokes orders (R>98%) and high transmission at 532 nm (T~90%). Best overlap between the pump waist and the resonator mode was achieved for high reflector and output coupler mirrors of concave radii-of-curvature ~200mm spaced as closely as practicable. We used a variety of output couplers having transmission values at each Stokes wavelength as listed in Table 1 below.

The Stokes and pump powers were measured using power meter (Ophir). The pump powers and resulting calculated efficiencies are based on the measured pump power transmitted by the input mirror and incident on the Raman crystal. A small amount of residual 1064nm fundamental output is produced by the pump module (typically <25mW depending on the chosen output coupler), however, this is removed from all the output power measurements. The spectral content of the Stokes output was measured using a fibre-coupled spectrometer (Spectra-Array, Ocean Optics). Transmission values of the output couplers were measured using a spectrophotometer (Cary5, Varian).

 

Fig. 1. The experimental arrangement showing the folded Raman resonator, 532nm pump laser, half-wave plate, and focusing lens. (O/C output coupler, HR high reflector)

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3. Results

The spectral content and output power from the Raman resonator is highly dependent upon the transmission characteristics of the resonator mirrors. We investigated a range of mirror radii-of-curvature and transmission characteristics in order maximize the output at each of the first 4 Stokes orders. The performance, spectral content and output coupling values for maximum output power at each target Stokes order are summarized in Table 1. As a general rule, high output power and efficiency was obtained for output couplers having transmission (at the target wavelength) in the range 5–50% and highly reflecting at shorter wavelengths. For the first and second Stokes target wavelengths, up to 400mW total output power was obtained with overall conversion efficiencies (25–40%) similar to that achieved previously in references [2,6]. The output power and efficiency for the third and fourth Stokes were approximately a factor of two lower (~200 mW at 10–20% efficiency).

For many of the investigated output couplers and indeed several of operating conditions listed in Table 1, the output consisted of more than the target Stokes line. The pattern of spectral content differed depending on the crystal orientation. For the crystal orientation corresponding to the larger Stokes shift, i.e., the pump polarization parallel to the crystal a-axis, the output typically contained a small fraction of the next Stokes order in addition to the target wavelength. For the crystal orientation corresponding to the smaller Stokes shift (i.e., the pump polarisation parallel to the crystal c-axis), we observed simultaneous generation of Stokes output for both 768cm^-1 and 902cm^-1 Stokes shifts. For the first-Stokes output coupler, almost equal output at 555nm and 559nm was generated. For the output couplers corresponding to maximum output at the third and fourth Stokes we observed not only observed multiple Stokes orders (579nm, 606 nm, 636 nm and 669nm) but also the first-Stokes at 902cm^-1 down-shifted by 768cm^-1 multiple orders (i.e., to wavelengths 583 nm, 612 nm and 641 nm). A notable variation to this pattern was obtained at the second-Stokes for which very pure output (>99%) at 579 nm was obtained. The mixing of the two Stokes shifts reflects their comparable gain in this orientation of the crystal [7].

For many applications, the conversion efficiency to a specific wavelength is an important parameter of interest. The highest single-wavelength output power and efficiency was 588 nm which corresponds to the second-Stokes for the pump polarization aligned with the a-axis. Of the total output power 396 mW, >90% of the output was at 588nm (i.e., >355mW) and the remainder at 621nm. By using other output couplers, more than 200mW was obtained at 579 nm and 559 nm.

Table 1. The output performance, spectral content and output couplings for maximum overall efficiency at each of the first 4 Stokes orders and for the two orthogonal orientations of the Raman crystal

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An overall glance at the results in Table 1 reveals that the efficiencies and output powers are on-average much higher for the pump polarized along the a-axis, which is the orientation yielding highest Raman gain [7]. On the other hand, the thresholds appear to follow no clear dependence on the pump polarization and are thus deduced to be strongly dependent on the output coupling values. The thresholds were also sensitive to the cavity length and were noted to decrease to <260mW when using a more compact linear resonator. However, higher maximum output power and efficiency was obtained using the folded resonator since our angled input mirror had higher transmission at the pump wavelength (80% cf. 67% for the linear cavity) compared to our normal incidence input mirror.

Above threshold the Stokes output power is essentially linear for each investigated output coupler as shown for example by the output characteristic curves for second Stokes output plotted in Fig. 1. The maximum slope efficiency achieved was 68% for the first Stokes output with the crystal orientation for 555nm. This slope efficiency is higher than that achieved in the external resonator configurations using Ba(NO₃)₂ for either a 532 nm pump at 10–30 Hz (<50%) [6], or a 1064nm pump at 4 kHz (63%) [2].

The measured spatial characteristics for Stokes output revealed that the output for all conditions is TEM₀₀ mode with M² beam quality factor approximately 1.5. The near field profiles are peaked on-axis and single lobed. By way of example, the near-field beam profile for second-Stokes output (pump a-axis polarized) at full pump power is shown in Fig. 2. The beam quality and near field profile are essentially unchanged in the investigated pump power range.

 

Fig. 1. Output power dependence on the 532nm pump power incident on the Raman crystal for the resonator configurations corresponding to maximum second-Stokes output (both crystal orientations shown).

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Fig. 2. Near field beam profile for second-Stokes output (ie., largely 589nm output) at maximum pump power.

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3. Discussion

The significant output powers obtained at a large number of wavelengths obtained spanning the red, orange and yellow spectral region demonstrates that external Raman resonators based on KGd(WO₄)₂ are wavelength-versatile as well as efficient. Single wavelengths are obtained without inserting wavelength-selecting elements into the resonator for some of the selected output couplers and Raman crystal orientations. In order to achieve efficient and spectrally pure output, it was generally found that the output coupling at the target wavelength was in the range 5–50%, while being highly reflective for the shorter wavelength orders and highly transmissive for the longer wavelength orders. Custom-designed mirror coatings, dispersive cavity elements or the use of absorptive elements at higher Stokes order wavelengths [9] may also assist in generating spectrally pure output. With careful design it is expected that spectrally pure output is possible for all the observed wavelengths which include integer multiples of the two Stokes shifts of KGd(WO₄)₂ and some mixtures of the two Stokes shifts.

Although Table 1 present results for maximum output power at the target Stokes order, they also highlight the capability for efficient multi-wavelength generation. Indeed, we observed output at two wavelengths simultaneously for most of the output couplers used in this study, and for some output couplers up to 5 wavelengths. Multi-wavelength operation is of interest for applications in countermeasures. Moreover, by using second harmonic generation or sum frequency mixing a number of ultraviolet wavelengths can be accessed in the range 275–300 nm, which are of interest for UV lidar and biosensing.

Though the achieved overall efficiencies for visible generation (up to 40%) are commensurate with that obtained previously [2,6], several avenues exist for further increasing overall efficiency. As only modest pump powers have been used here, (i.e., <3 times the threshold for Raman laser oscillation), increased efficiencies are likely to be achieved at higher pump powers. Indeed we observe no saturation in output power or degradation in the beam profile when increasing the input power up to the present power limit of our pump module (Fig. 1). These observations and the resulting potential for further power scaling reflect the excellent thermal properties of KGd(WO₄)₂. The use of a simple two-mirror linear resonator also presents an avenue for decreasing threshold and increasing overall efficiency. The reduced thresholds we observed for a shorter linear cavity most likely resulted from the increased number of round-trips available within the duration of the pump pulse. Thus for a normal incidence input mirror having improved spectral characteristics, higher output powers and overall efficiencies are likely to be obtained. Finally, similar improvements may also be obtained by ensuring a resonator mode and pump waist overlap better than that achieved here due to a slight assymmetry in the far-field profile of the 532 nm pump laser.

4. Conclusion

We have demonstrated single and multi-wavelength output covering more than 8 wavelengths in red, orange and yellow using an external KGd(WO₄)₂ Raman resonator. No saturation in the output power is observed which suggests that Stokes output will scale well for pump powers >1 W with projected overall conversion efficiencies >50% and output powers rivalling that of intracavity configurations. The KGd(WO₄)₂ Raman resonator thus provides a simple and efficient frequency-downshifting add-on well suited to multiwatt second-harmonic Nd pump sources which are now widely available.