Generation and Modulation of Phase Difference of Output Intensities in a Feedback Nd:YAG Laser with an Extracavity Waveplate Rotated

External anisotropic feedback effects on the phase difference behaviour of output intensities in a microchip Nd：YAG laser are presented. By rotating a quarter wave plate placed in the external cavity, the angle between laser initial polarization direction and o-axis of the wave plate is tuned from -45°to 45°, which results in variable extra-cavity birefringence along two orthogonal detection directions. With only one optical path and one wave plate, laser intensities of the two orthogonal directions, both modulated by the external cavity length, are output with a tunable phase difference, which can be continuously changed from zero to twice as large as that of the waveplate. Experimental results as well as a theoretical analysis based on Fabry-Perot cavity equivalent model and the refractive index ellipsoid, are presented. The potential applications of this phenomenon are also discussed.     

Tunable dual wavelength emission and bandwidth narrowing of a laser diode array with a simple external cavity

We report on the performance of a commercial, non-AR-coated diode laser bar of 20 W output power, consisting of 20 emitters, which uses an external cavity to achieve tunable, dual-wavelength emission. The separation between the wavelength peaks can be continuously tuned from 0 nm to 5 nm. An output power of 2.9 W is achieved at 3.7 nm peak-to-peak separation and higher powers are achieved for less separation. This is the highest dual wavelength output power reported so far using a standard diode laser array. PACS 42.55.Px; 42.60.-v; 42.60.Da; 42.60.Fc     

Radially polarized laser beam from a Nd:YAG laser cavity with a c-cut YVO4 crystal

We demonstrate the generation of a radially polarized beam by simply inserting an undoped c-cut YVO₄ crystal into a Nd:YAG laser cavity. In a hemispherical cavity, the cylindrically symmetric, positive birefringence of the YVO₄ crystal extends the stability limit of the cavity length for an extraordinary ray (radial polarization) compared to an ordinary one (azimuthal polarization). By adjusting the cavity length, a radially polarized beam with an output power up to 1 W was selectively obtained. In addition, a higher-order transverse mode was also generated by arranging the cavity design. The method demonstrated in this paper can be readily applied to laser systems with an isotropic laser medium. PACS 42.60.Da; 42.25.Lc; 42.25.Ja     

External-cavity birefringence feedback effects of microchip Nd:YAG laser and its application in angle measurement

External-cavity birefringence feedback effects of the microchip Nd:YAG laser are presented. When a birefringence element is placed in the external feedback cavity of the laser, two orthogonally polarized laser beams with a phase difference are output. The phase difference is twice as large as the phase retardation in the external cavity along the two orthogonal directions. The variable extra-cavity birefringence, caused by rotation of the external-cavity birefringence element, results in tunable phase difference between the two orthogonally polarized beams. This means that the roll angle information has been translated to phase difference of two output laser beams. A theoretical analysis based on the Fabry--Perot cavity equivalent model and refractive index ellipsoid is presented, which is in good agreement with the experimental results. This phenomenon has potential applications for roll angle measurement.     

A new laser scheme for energy scaling with a composite six-mirror cavity

We have developed a novel and compact six-mirror cavity configuration to combine four multimode laser modules intracavity for energy scaling. Compared to the conventional 2×N channel scaling schemes, this close-ended six-mirror cavity is simpler, more stable and more practical. Diode-side-pumped rod laser geometry that is appropriate for beam combination is employed in the branch laser module. The characteristics of the output energy and the beam quality are investigated experimentally. With the six-mirror cavity, the output brightness is significantly improved compared to single F–P cavity laser. A single beam output exceeding 453 mJ with 165 μs duration is achieved, with the combination efficiency of 90.7%. PACS 42.60.Da; 42.60.Jf; 42.60.By     

Output response in orthogonal directions of a He–Ne laser caused by birefringent-external-cavity feedback

The output responses of phase, amplitude, and polarization of light intensity of a He–Ne laser with the birefringent external cavity are presented. All phenomena are observed in two orthogonal directions related to the birefringent element’s orientation, and are studied under weak optical feedback. Experiments are carried out in five available detecting positions. It is found that laser intensities are modulated in the two orthogonal directions simultaneously, with a phase difference dominated by the phase retardation of the birefringence element. The modulation amplitudes are different in the two directions. A special polarization phenomenon is also observed and discussed. This research may provide a potential novel measuring principle for transparent birefringent materials.     

Dual-wavelength bandwidth-narrowed output of a high-power diode laser using a simple external cavity

Using an external cavity consisting of an etalon and a mirror, dual-wavelength operation of a high-power broad-area multi-stripe diode laser is achieved. The reflection of the etalon is used as the output beam of the system. The free-running bandwidth of the laser diode is about 2.0 nm. At dual-wavelength operation, the bandwidth of each wavelength component is narrowed to about 0.07 nm, while the space between them is 1.65 nm, determined by the FSR of the etalon. We obtain an available dual-wavelength output power of 2.0 W at the drive current of 6.5 A. The power ratio of the components at two different wavelengths can be changed by changing the temperature of the diode laser. To tune the wavelength of the dual-wavelength output, the temperature of the laser diode and the tilt angle of the etalon are changed simultaneously PACS 42.55.Px; 42.60.Fc; 42.60.Da     

Sensitive birefringence measurement in a high-finesse resonator using diode laser optical self-locking

We demonstrate a new method to measure weak birefringence of dielectric mirrors with excellent spatial resolution and sensitivity (<10^-7 radians). We exploit a well-known optical feedback scheme for line-width narrowing and frequency locking of a diode laser to a high-finesse cavity. Feedback comes from the intracavity field which builds up at resonance, selected by its change in polarization with respect to the incident field. This change, due to the residual birefringence of the cavity mirror coatings, was already exploited for birefringence measurements using an active laser-locking scheme. Here we measure the optical feedback rate as a function of rotation angle of one of the cavity mirrors (around the cavity axis). A stable feedback signal is obtained since the laser, as soon as it locks to a cavity resonance, effectively behaves as a monochromatic source. By fitting the data with a theoretical expression, we determine quantitatively the local birefringence vectors of both mirrors, which are around 10^-6 radians. Our scheme is simple, works with cavities of very high finesse (F∼10⁵), and is promising for measuring birefringence in gases induced by external fields. Received: 18 July 2001 / Final version: 14 March 2002 / Published online: 8 May 2002     

Extended temperature tuning of an external cavity diode laser

We describe the construction and operation of an external cavity diode laser in which the diode is cooled to -45 °C. This technique allows us to pull the wavelength of a nominally 782 nm diode to operate at 766.7 nm, a change of over 15 nm. The important aspects of our design are its ease and low cost as compared to other designs of changing the operating wavelength of laser diodes. We provide background information on thermal design with multiple TECs so the reader can extend this example for use in their own applications. PACS 42.60.By; 42.62.Fi; 32.80.Pj     

Dual-wavelength external cavity laser with a sampled grating formed in a silica PLC waveguide for terahertz beat signal generation

We propose and demonstrate a dual-wavelength external cavity laser (ECL). In our design, a Fabry–Pérot laser diode (FP-LD) is hybrid-integrated with a sampled Bragg grating written in a silica planar lightwave circuit (PLC). The grating selects two laser wavelengths that share the same laser cavity. The dual-wavelength oscillation with a side mode suppression ratio >32 dB has been demonstrated experimentally. Experimental results indicate that the hybrid-integrated dual-wavelength ECL exhibits strong dual-wavelength emission corresponding to beat frequency of 1 THz. Simulations of this system also indicate good mutual coherence of the two modes and stability of the 1 THz beat signal. PACS 42.79.Dj; 42.60.By; 42.55.Px     

Self-mixing interference effects in orthogonally polarized dual-frequency Nd:YAG lasers at different feedback levels

The self-mixing interference effects are investigated in birefringence dual-frequency Nd:YAG lasers at different optical feedback levels. For weak feedback level, both of the intensities of the two modes are sinusoidally modulated by external cavity length with a feedback period of half laser wavelength. While these two modes can only coexist in a part of feedback period at moderate feedback level, i.e., one mode reaches its maximum intensity, the other one extinguishes. At high feedback level, complete polarization switching (one mode oscillating, the other one extinguishing) between two polarized modes is observed, and every switching corresponds to quarter-wavelength variation of external cavity length. Furthermore, two feedback schemes, i.e., isotropic optical feedback (IOF) and polarized optical feedback (POF) are taken into account. The theoretical analysis based on a model of two-mode-coupled laser is presented, which is in good agreement with experimental results. Our results can advance the research of self-mixing interferometer of orthogonally polarized dual-frequency lasers.     

Optimization of coupled confocal cavities for an injection-seeded,discharge-excited KrF laser system

The output energy and beam divergence of an injection-seeded KrF laser system, operating at 248 nm, were measured for twelve different coupled confocal unstable cavity arrangements, as a function of the relative timing between the slave and master oscillator lasers as well as of the cavity magnification. The output energy was found to be strongly dependent on both the cavity magnification and the time delay, although the beam divergence shows a less sensitive dependence. An optimized output of 2.2 J in 23 ns with a divergence of 38 μrad was achieved, representing a brightness several times larger than any previously reported for a system of this type. PACS 42.55.Lt; 42.60.Da; 42.60.Jf     

Laser Intensity Variation in Amplitude and Phase Induced by Elliptically Polarized Feedback

The laser output characteristics under elliptically polarized optical feedback effect are studied. Elliptically polarized light is generated by wave plate placed in the feedback cavity. By analyzing the amplitude and phase of the laser output in the orthogonal direction, some new phenomena are firstly discovered and explained theoretically.Elliptically polarized feedback light is amplified in the gain medium in the resonator, and the direction perpendicular to the original polarization direction is easiest to oscillate. The laser intensity variation in amplitude and phase are related to the amplified mode and the anisotropy of external cavity. The theoretical analysis and experimental results agree well. Because the output characteristic of the laser has a relationship with the anisotropy of the external cavity, the phenomenon also provides a method for measuring birefringence.     

Ultraviolet diode laser system based on the resonant optical feedback method with the capability of fast continuous sweep

An ultraviolet laser diode system stabilized to an external Fabry–Perot cavity by using the resonant optical feedback method is described. Both wide continuous sweep with fast response and drift compensation for long term operation are realized simultaneously by performing feedforward control along with feedback control of the system parameters. By utilizing the capability of synchronized sweep realized by the feedforward control, the modulation of the output frequency, due to dither for lock-in feedback control of the system parameters, is almost perfectly canceled. The method described here is useful for developing diode laser systems for practical use with such features as small short term fluctuation, long-term operability and fast continuous sweep. PACS 42.55.Px; 42.60.By; 32.80.Pj     

Axial mode tuning of a single frequency Yb:YAG thin disk laser

We describe an arrangement for an Yb:YAG thin disk laser, which enables narrow bandwidth operation in single-frequency mode at freely selectable wavelengths within the broad tuning range of the laser. This is facilitated by a combination of a double-stage birefringent filter and an etalon inside the laser cavity. We investigate the wavelength selection characteristics of the single elements as well as their combination. A simple procedure is implemented for a computer-based automation of wavelength tuning. The reflectivity of the partially reflecting resonator mirror is optimised, and the laser pump power is adapted for best tuning performance. Single-frequency emission is achieved in a frequency range of 9.75 THz (wavelength range 1020 nm to 1055 nm). Each axial laser mode in this range can be selected individually. The axial mode separation of 0.47 GHz corresponds to wavelength steps of 1.7 pm at 1030 nm. PACS 42.55.Xi; 42.60.Fc; 42.60.Lh     

Widely tunable mode-hop free external cavity quantum cascade laser for high resolution spectroscopic applications

An external cavity (EC) quantum cascade laser (QCL) configuration with the thermoelectrically cooled gain medium fabricated using a bound-to-continuum design and operating in continuous wave at ∼5.2 μm is reported. The EC architecture employs a piezo-activated cavity mode tracking system for mode-hop free operation suitable for high resolution spectroscopic applications and multiple species trace-gas detection. The performance of the EC-QCL exhibits coarse single mode tuning over 35 cm^-1 and a continuous mode-hop free fine tuning range of ∼1.2 cm^-1. PACS 07.07.Df; 42.55.Px; 42.62.Fi; 42.60.Fc     

Development of a highly stable Yb:YAG thin disk pulsed green laser for high power Ti:sapphire based amplifier at 100 kHz repetition rate

We have developed a pulsed Yb:YAG thin disk green laser for pumping a 100 kHz and 10 W femtosecond laser. We optimized numerically and experimentally the open duration of the acousto-optic (AO) Q-switcher and the internal SHG method, generating stable output energy at a 100 kHz repetition rate. We demonstrated a 515 nm output power of over 80 W with a pulse-to-pulse stability of less than 1% RMS with an LD power to green power conversion efficiency of 24%. With the developed green laser, we obtained 16 W of a Ti:sapphire laser in a cavity dumped mode. PACS 42.55.Xi; 42.60.Gd; 42.60.By; 42.65.-k; 42.65.Re     

Efficient blue light generation by frequency doubling of a broad-area diode laser in a compact external cavity

An antireflection-coated broad-area laser diode is frequency stabilized in a compact unstable external cavity that was designed off-axis for transversal mode selection. The external cavity laser yielded up to 1 W near-diffraction-limited and tunable light centered around a wavelength of 970 nm. This light was frequency doubled by the use of periodically poled magnesium doped lithium niobate in a single-pass setup. With a 4-cm-long bulk crystal 57.5 mW of blue light could be generated out of 850 mW of infrared light resulting in a conversion efficiency of 6.7%. By using a 1.2-cm waveguide crystal it was possible to couple up to 64% of the infrared light into a 3.5 μm×5 μm channel and 142 mW of blue light at 488 nm could be obtained. An internal conversion efficiency of 44.1% and a wall-plug efficiency of 5.9% were achieved. Furthermore, the blue light was tunable over more than 5 nm. PACS 42.55.-f; 42.55.Px; 42.60.Da     

Theoretical investigation on the intracavity Doppler effect in continuous wave swept-cavity ringdown spectroscopy

We theoretically investigate the spectral measurement errors that are apt to occur in continuous wave (CW) cavity ringdown (CRD) techniques suffering intracavity Doppler shifts. In the typical CW CRD scheme based on a cavity sweep operation for the resonant coupling of a probe laser into a ringdown cavity, the intracavity probe light detunes gradually over time, carrying time-dependent loss information of an absorption feature. Frequency-drifting ringdown signals are theoretically modelled and found to result in erroneous absorption spectra that exhibit the frequency shift of absorption profiles as well as linewidth broadening. PACS 42.62.Fi; 42.60.Da; 42.25.BS     

基于1556 nm光纤激光器频率分裂效应的应力测量

光学元件在红外波段的应力-光学常数是众多光学系统关心的问题之一.本文提出一种基于1556.16 nm掺铒光纤激光频率分裂效应的光学玻璃内应力致双折射测量方法.选择平面介质膜腔镜和光纤光栅(FBG)构成线形半开放式谐振腔,并分析了光纤自身弯曲引入谐振腔内的双折射.将待测光学玻璃附带力传感结构放置在谐振腔内,结合Jones矩阵传递方程得到了外载荷所致双折射与空腔双折射的叠加模型.对光学玻璃的载荷从0逐级递加到20 N,内腔的频率分裂量增加,根据双折射叠加模型和频率分裂原理解出应力与频率分裂量的对应关系,且该结果可溯源到基本物理量—波长.实验结果表明,系统灵敏度为22060 Pa/nm,线性度为99.44%,可广泛应用于红外波段的光学元件双折射精确测量.