Semiconductor saturable absorber mirrors supporting sub-10-fs pulses

Received: 18 March 1997/Revised version: 12 May 1997     

A model for the optimization of double-clad fiber laser operation

Received: 27 October 1997/Revised version: 12 January 1998     

High-power broad-area diode lasers for laser cooling

Received: 1 April 1996/Revised version: 24 September 1996     

1.24-μm cascaded Raman laser for 1.31-μm Raman fiber amplifiers

Received: 10 November 1997     

Standard and phase-matched grazing-incidence and distributed-feedback FIR gas lasers

In this paper we review theoretical and experimental studies on optically pumped 496 m CH₃F DFB lasers of different configurations, including grazing-incidence arrangement and phase matching by a gap in the periodic structure. These configurations combine the simple tuning mechanism of grazing-incidence systems with the high frequency selectivity of DFB. Our theoretical considerations based on coupled-wave theory are concerned with the dispersion relations and resonance conditions of standard and phase-matched DFB and grazing-incidence gas lasers. We have succeeded in calculating the relevant TM coupling coefficients for lasers with rectangular periodic waveguides. For laser cavities with various continuous gratings we have measured the resonant heights and tuning angles of the laser oscillations of first- and second-order DFB. We have found good agreement with theoretical resonance conditions. In order to improve the mode selectivity and to attain single longitudinal mode operation, which is a requirement for semiconductor lasers in many applications, we have introduced variable gaps in the center of the gratings. These provide phase matching and gap modes. We have compared the measured gap modes with our theory and found agreement in specific cases, where the phase-matched cavity implies single-mode laser operation. Our results on standard and phase-matched DFB cavities promise an improvement of the performance of phase-matched semiconductor lasers with respect to small bandwidth and optimized output power.     

Energy theory of waveguide distributed feedback gas lasers

In this paper systematic numerical studies of the nonlinear operation of a waveguide distributed feedback gas laser equipped with a metal waveguide of rectangular cross section are presented. An approximate formula relating small-signal gain with the output power and the parameters of the structure for this kind of lasers is derived. Characteristics revealing saturation effects for weak as well as strong coupling regions are presented.Academic guest from the Institute of Microelectronics and Optoelectronics, Technical University, Warsaw, Poland     

Compact mid-infrared trace gas sensor based on difference-frequency generation of two diode lasers in periodically poled LiNbO3

3 (PPLN) crystal pumped by two single-frequency diode lasers. A maximum DFG power of 1.6 μW at 3.6 μm was generated with a pump power of 61.4 mW at 832 nm and a signal power of 41.5 mW at 1083 nm incident on a 19-mm-long PPLN crystal, which corresponds to a conversion efficiency of 335 μW W^-2 cm^-1. Received: 16 June 1998     

Precise measurement of methane in air using diode-pumped 3.4-μm difference-frequency generation in PPLN

3 volume multi-pass absorption cell with an 18-m path length. The methane mixing ratio was determined by comparing the direct optical absorption measured in the sample with that measured in a reference gas at 100 torr and room temperature. Relative accuracy of better than 1 ppb (parts in 10⁹, by mole fraction) was achieved in measurements of natural air that contained 1700–1900 ppb methane. The typical measurement time for each sample was 60 seconds. The accuracy was limited by residual interference fringes in the multi-pass cell that resulted from scattering. Without the use of reference samples, the relative accuracy was 20 ppb; it was limited by the long-term reproducibility of the spectroscopic baseline, which was affected by drift in the optical alignment coupled to changes in the ambient temperature. This work demonstrates the use of diode-pumped difference-frequency generation (DFG) in PPLN in a high-precision infrared spectrometer. Compact, room-temperature solid-state gas sensors can be built based on this technology, for accurate real-time measurements of trace gases in the 3–5 μm spectroscopic region. Received: 2 October 1996     

High-average-power diode-pumped femtosecond Cr:LiSAF lasers

Received: 27 February 1997/Revised version: 14 April 1997     

Compact laser difference-frequency spectrometer for multicomponent trace gas detection

2 O), carbon dioxide (CO₂), and sulfur dioxide (SO₂). Real-time detection of CO, N₂O, and CO₂ was performed in open air over a path length of 5 to 18 m. The feasibility of DFG spectroscopic measurement of the ¹³C/¹²C and ¹⁸O/¹⁷O/¹⁶O isotopic ratios in atmospheric carbon dioxide was also investigated. We report what to our knowledge is the first simultaneous spectroscopic measurement of all three isotopes of oxygen in ambient CO₂. Received: 14 August 1997/Revised version: 3 November 1997     

Ultrashort-pulse sources based on single-mode rare-earth-doped fibers

An overview of ultrashort-pulse sources based on single-mode rare-earth-doped fibers is given. A wide range of pulse-generation schemes comprising mode-locked fiber lasers, parametric pulse sources and hybrid diode-fiber amplifier sources are discussed. Both actively and passively mode-locked fiber lasers are described and their specific merits and operation regimes are elucidated. Techniques for improving the spectral quality and the output powers of diode-based systems based on amplification in rare-earth-doped fibers are also reviewed. Finally, applications are discussed and directions for future research are indicated.     

Active synchronization of two mode-locked lasers with optical cross correlation

Two mode-locked Ti:sapphire lasers of different wavelengths were precisely synchronized by a simple feedback system employing sum-frequency generation (cross correlation). When the timing error exceeded the pulse duration, the periodic bunch of the sum-frequency pulse was used for rough timing adjustment. Using cross correlation with a stretched pulse, we struck a balance between wide locking range and sensitive timing detection. When the two lasers were well-synchronized, we obtained a continuous cross-correlation pulse train for 3 min. The holding time of the laser synchronization was extended to over one hour by adding a motorized stage to the PZT-mounted cavity mirror. We estimated the rms timing jitter between the two lasers by a scanning cross-correlation measurement. We confirmed that the rms timing jitter of the two lasers during 1.8 s was 28 fs. Received: 30 January 2002 / Revised version: 14 June 2002 / Published online: 8 August 2002     

Detection of formaldehyde using mid-infrared difference-frequency generation

3 (PPLN) is reported. Minimum detectable concentration of 30 ppb was achieved with a compact, portable room-temperature gas sensor configured for formaldehyde (H₂CO) detection. This sensitivity, coupled with high selectivity and long term stability, is sufficient for various environmental applications. Received: 29 September 1997     

Optical and spectroscopic characterization of Nd-doped aluminosilicate fiber preforms made by the MCVD method using chelate delivery

4 F_3/2 level of Nd³⁺ in aluminosilicate optical fiber preforms after 807-nm excitation are reported in the temperature range from 15 °C to 500 °C for Nd₂O₃ concentrations in the range from 0.2 to [%wt]0.9 and Al₂O₃ concentrations of between [%wt]3.8 and [%wt]8. At room temperature it is observed that for concentrations of less than [%wt]0.6 Nd₂O₃ the fluorescence decay is approximately exponential; for the higher Nd₂O₃ concentrations the decay is increasingly bi-exponential. We have identified a “slow” (≈500 μs) and a “fast” (≈100 μs) decay time; the slow decay is attributed to isolated ions and the faster decay to energy transfer between homogeneously dispersed ions. The measured 1/e folding fluorescence decay time τ decreases slowly at a constant rate of -0.08±0.007 μs/K for [%wt]0.9 Nd₂O₃. It is shown that increasing the Al₂O₃ concentration tends to suppress this temperature quenching effect. A radiative decay time of 640 μs is obtained by Judd–Ofelt analysis. Received: 24 October 1997/Revised version: 2 February 1998     

Comparison between c-cut and a-cut Nd:YVO4 lasers passively Q-switched with a Cr4+:YAG saturable absorber

Comparison between c-cut and a-cut Nd:YVO₄ microchip lasers passively Q-switched with a Cr⁴⁺:YAG saturable absorber is experimentally made. The lower emission cross section of the c-cut Nd:YVO₄ crystal can enhance the passive Q-switching effect to produce a peak power 10 times higher than that obtained with the a-cut crystal. The experimental result further reveals that a c-cut Nd:YVO₄ crystal is a very convenient material for short-pulse (sub-nanosecond) and high-peak-power (>10 kW) lasers. Received:10December2001/Revisedversion:22January2002 / Published online: 14 March 2002     

Solitons and vortices in lasers

We give in this article the mathematical background for pattern formation in nonlinear active resonators, elucidating the relation of optics with other fields of physics, and demonstrate experimentally the existence, properties, and dynamics of: (i) vortices in lasers, (ii) bright spatial solitons in lasers with saturable absorber, and (iii) spatial solitons in degenerate parametric mixing. All these structures are by definition bistable so that they are potentially useful for parallel optical information processing. Received: 8 June 1998     

Phase conjugation in a continuous-wave diode-pumped Nd:YVO4 laser

We demonstrate phase conjugation and dynamic holography by degenerate four-wave mixing in a continuous-wave diode-pumped Nd:YVO₄ laser for the first time. A phase-conjugate reflectivity of 1.4% is obtained with a 12 W diode power. The experimental results and the optimizing parameters, such as the cavity mirror reflectivities, are compared with results from a steady-state theoretical analysis. Received: 6 April 1999 / Published online: 24 June 1999     

Pulse formation in synchronously pumped infrared dye lasers

Pulse formation in synchronously pumped dye lasers with fast relaxing dyes is treated by computer simulation. The influence of spontaneous emission and chirp, as well as cavity mismatch, on the pulse characteristics and stability of generation are discussed.     

Measurement of the residual birefringence of interferential mirrors using Fabry-Perot cavity

Received: 23 September 1996/Revised version: 10 January 1997     

Soliton-like pulse-shaping mechanism in passively mode-locked surface-emitting semiconductor lasers

We discuss a mechanism that allows the formation of nearly transform-limited soliton-like pulses in passively mode-locked optically pumped external-cavity surface-emitting semiconductor lasers. It involves the interplay of positive dispersion and the nonlinear index changes in gain medium and saturable absorber, while ordinary solitons are based on dispersion and the Kerr effect. The obtained quasi-soliton pulses share some of the properties of ordinary solitons (in particular, their stability and near bandwidth-limited nature), while other properties are different. In particular, the pulse duration scales with the square root of the cavity dispersion, and an excessive drift of the laser wavelength must be avoided by proper design. Received: 23 April 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1059, E-mail: Paschotta@iqe.phys.ethz.ch