A 200-mW, 205-nm, quasi-CW, deep ultraviolet laser source
K. F. Wall, J. S. Smucz, Y. Isyanova, B. Pati, and, P. F. Moulton
Q-Peak, Inc., 135 South Road, Bedford, MA 01730
(781) 275-9535, kwall@qpeak.com
J. Manni
JGM Associates, Inc., 6 New England Executive Park, Burlington, MA 01830
(781) 272-6692, jgmanni@jgma-inc.com
Abstract: A quasi-cw, deep ultraviolet source has been developed, producing >200 mW of 205‑nm radiation. The source consists of a mode-locked 16-W 1047-nm master-oscillator/power-amplifier, a synchronously-pumped optical parametric oscillator, and three non-linear conversion stages.
Ó2001 Optical Society of America
OCIS codes: (140.3480) Lasers, diode-pumped; (140.4050) Mode-locked lasers; (190.4160) Multiharmonic generation
1. Introduction
Considerable effort has been devoted to developing shorter-wavelength laser sources for various optical processes and material processing. As an example, laser sources for photolithographic techniques have been pushed to shorter wavelengths in order to achieve higher resolution, in an effort to keep up with Moore’s Law.
For some applications, a cw or quasi-cw source of sufficiently high repetition rate is desirable. We have constructed a deep-ultraviolet (UV) source consisting of a 100-MHz repetition rate, mode-locked master-oscillator followed by two Multi-Pass Slab (MPS) gain modules to generate as much as 16 W of M2 ~ 1.2, 1047-nm radiation. We have investigated several methods to generate deep-UV and report here on the approach that generated the highest power. Our design is based on the use of three non-linear conversion stages and a synchronously-pumped optical parametric oscillator (OPO). The overall system concept is shown schematically in Fig. 1 and the details of the system are described below. Except for one case, the nonlinear processes were designed to operate with non-critical phase matching (NCPM), in order to maximize conversion efficiency with the relatively small amount of nonlinear drive available from the quasi-CW source.
2. The Nd:YLF Master Oscillator
The system master oscillator is a modified Time-Bandwidth Products model GE-100 mode-locked laser. The GE-100 oscillator is a passively mode-locked, 1047-nm, Nd:YLF laser producing ~5 ps pulses with a repetition rate of 100 MHz. The primary modification of the oscillator was the use of a higher-power pump diode laser to increase the average output power from the standard 100-mW level to ~700 mW.
Fig. 1 The system design. The indicated wavelengths are in nm.
2.1 The Nd:YLF Amplifiers
The amplifier stages include a double-pass preamplifier followed by a single-pass power amplifier, both consisting of Nd:YLF MPS gain modules pumped by 800-nm diode lasers [1]. Typically the output power of the double-pass preamplifier is ~6 W at 40 W of pump power for an overall signal gain of ~15. The output of the preamplifier was further amplified to 16 W in a second Nd:YLF gain module, the power amplifier. The signal beam was passed through the power amplifier only once since the input intensity is on the order of the Nd:YLF saturation intensity (2 kW/cm2). The beam quality was measured to be 1.24 and 1.14 for the horizontal and vertical axes, respectively. Optics following the amplifier were used to remove the astigmatism and circularize the output beam from the power amplifier.
2.2 Non-Linear Conversion Stages
Starting with the 1047-nm mode-locked master-oscillator/power-amplifier (MOPA) output, second harmonic generation was used to halve the fundamental wavelength. The 1047-nm MOPA system output was frequency doubled using a 20-mm-long, NCPM, lithium triborate (LBO) crystal with an aperture of 2.25 x 2.75 mm. The crystal was placed in a non-sealed oven and heated to a temperature of ~170 ºC.
The 1047-nm fundamental beam was focussed into the LBO crystal with a spherical lens. From the measured beam parameters of the 1047 nm beam, the calculated beam radii (1/e2 ) at the focus of the lens were 84 and 72 mm for the horizontal and vertical dimensions, respectively. A spherical lens was used to recollimate the output beam from the LBO crystal. With 16.0 W of fundamental power, as much as 8.4 W of 524 nm power was produced.
The 524-nm beam was split into two legs: an optical parametric oscillator leg and a fourth harmonic generation (4HG) leg. The output of these two legs were combined, using a dichroic mirror, and sum frequency mixing (SFM) was used to generate ~205-nm radiation. The 4HG leg contained a delay line that allowed us to synchronize the pulses from the OPO and 4HG legs at the SFM crystal. The physical layout of this design is shown in Fig. 2.
The fourth harmonic of the 1047-nm fundamental was generated by doubling the 524-nm beam in a critically phase-matched, cesium lithium borate (CLBO) crystal placed in a closed, N2 gas-purged oven heated to ~160°. The crystal aperture was 5 x 5 mm and the length was 10 mm. The crystal was cut for propagation at angles of q = 66.4° and f = 45°.
The 524-nm beam was focussed into the CLBO crystal with a cylindrical lens. From the measured beam parameters of the 524-nm beam, the calculated beam radii (1/e2 ) at the focus of the lens were 18 and 1053 mm for the horizontal and vertical (critical) dimensions, respectively. A cylindrical lens was used to recollimate the output beam from the CLBO crystal. With 4.6 W of 524 nm power, as much as 0.77 W of 262-nm power was produced.
Fig. 2. The system layout. The labels are as follows: 2HG – Second Harmonic Generation, OPO – Optical Parametric Oscillator, 4HG – 4th Harmonic Generation, and SFM – Sum Frequency Mixing.
2.2.1 Synchronously-Pumped Optical Parametric Oscillator
A synchronously pumped OPO was constructed using NCPM LBO. The q = 90°, f = 0°, crystal was 20-mm long with an aperture of 3 x 3 mm and was heated to ~160 °. With ~2.5 W of pump power, the maximum output power of the OPO was 550 mW at a signal wavelength of 940 nm. Only the signal power was measured since the idler was transmitted through the cavity mirrors. The slope efficiency and threshold calculated from a linear fit to the data are 29% and 0.67 W, respectively. The optical-to-optical efficiency of the OPO was 20% for conversion of the pump to signal and ~30% if the idler power is included
Due to the relatively high intracavity signal intensity in the resonator and the non-linear properties of LBO, significant self-phase-modulation was initially observed with low-transmission output couplers. This self-phase-modulation broadened the OPO output spectrum to the extent that it exceeded the acceptance bandwidth of the sum-frequency mixing stage. Group velocity dispersion compensating prisms were inserted in the OPO resonator to introduce negative group velocity dispersion and to compensate partially for the self-phase-modulation effects. The use of these prisms, greater output coupling (T=30%), and a ~200 nm etalon within the resonator resulted in a narrow (~300 pm) linewidth and increased frequency stability of the output. The pulsewidth of the OPO was measured (without the etalon in the cavity) with an autocorrelator and, assuming Gaussian pulse profiles, the calculated pulsewidth was 3.9 ps.
2.2.2 The Sum-Frequency Mixing Stage
The OPO wavelength was selected to provide NCPM in the CLBO SFM crystal. The outputs of the synchronously-pumped OPO and fourth harmonic legs of the system were synchronized in time by adjusting the delay line in the fourth harmonic leg. Next, they were combined using a dichroic mirror and spatially overlapped. Spherical lenses prior to the dichroic mirror focused the beams into a 5 x 5 mm aperture, 10 mm long SFM crystal. This crystal was maintained at ~200 °C in a closed, N2 gas-purged oven. Spot diameters, estimated by measuring the transmission of the beams through pinholes, were 40 mm and 100 mm for the fourth harmonic and OPO beams, respectively. The crystal was cut for propagation at q = 90° and f = 45°.
We obtained as much as 225 mW of 205 nm radiation. A wavelength of 942.4 nm appeared to be required for NCPM, which is ~30 nm higher than predictions using the data of Mori et al. [1] and ~15 nm lower than predictions using the temperature-dependent data of Umemura et al. [2].
3. Summary
The results of our development effort show that 200 mW of deep-UV, 205-nm output can be generated moderately efficiently utilizing a 1047-nm, mode-locked MOPA system and appropriate non-linear conversion stages. To obtain this level of deep-UV quasi-CW output with a moderate-power solid state laser, one must select the nonlinear crystals and wavelengths used with some care. In the design we report here, we have used borate materials exclusively and NCPM in all but one stage, to lower the drive requirements for efficient conversion.
4. References
[1] K. J. Snell, D. Lee, K. F. Wall, and P. F. Moulton, “Diode-pumped, high-power CW and modelocked Nd:YLF lasers,” OSA Trends in Optics and Photonics Vol. 34, Advanced Solid-State Lasers, H. Injeyan, U. Keller, and C. Marshall, eds. (Optical Society of America, Washington, DC, 2000) 55-59.
[2] Y. Mori, I. Kuroda, S. Nakajima, T. Sasaki and S. Nakai, “New nonlinear optical crystal: cesium lithium borate,” Appl. Phys. Lett. 67, 1818-1820 (1995).
[3] N. Umemura, K. Yoshida, T. Kamimura, Y. Mori, T. Sasaki, and K. Kato, “New data on the phase-matching properties of CsLiB6O10,” in OSA Trends in Optics and Photonics Vol. 26, Advanced Solid-State Lasers, M. M. Fejer, H. Injeyan, and U. Keller, eds., (Optical Society of America, Washington, DC, 1999) 715-719.
