Tunable external-cavity diode laser at 650 nm based on a transmission diffraction grating

A tunable external-cavity diode laser (ECDL) based on a transmission diffraction grating in a Littrow mount has been developed and characterized. A single-transverse-mode diode laser at 650 nm is used in an external-cavity configuration in which the transmission grating is used as a dispersive element to select the single longitudinal mode. The transmission diffraction grating is made with electron-beam lithography. A tunable true single-mode cw output power of >20 mW is obtained from the ECDL. The total wavelength tuning range is 12 nm, and the mode-hop-free continuous tunability is >20 GHz.     

Fast tuning of external-cavity diode lasers

We report on a method for characterizing the tuning capabilities of small-bandwidth external-cavity diode lasers. The step response of a piezo-driven Littman-Metcalf external-cavity diode laser and the compensation of its optoelectromechanical frequency response is investigated. When an approximately 50-V compensated Gaussian pulse is applied to the piezo element, a detuning of 13.6 GHz is observed. This modification of the laser is useful for several spectroscopical applications and as a tunable seed laser for lidar applications.     

Mode-locking external-cavity laser-diode sensor for displacement measurements of technical surfaces

A novel laser sensor for position measurements of technical solid-state surfaces is proposed. An external Fabry-Perot laser cavity is assembled by use of an antireflection-coated laser diode together with the technical surface. Mode locking results from pumping the laser diode synchronously to the mode spacing of the cavity. The laser cavity length, i.e., the distance to the measurement object, is determined by evaluation of the modulation transfer function of the cavity by means of a phase-locked loop. The mode-locking external-cavity laser sensor incorporates a resonance effect that results in highly resolving position and displacement measurements. More than a factor-of-10 higher resolution than with conventional nonresonant sensing principles is achieved. Results of the displacement measurements of various technical surfaces are reported. Experimental and theoretical investigations are in good agreement.     

采用DBR LD的正弦相位调制激光干涉仪

分析影响正弦相位调制半导体激光干涉仪测量精度和系统分辨力的因素,提出了用分布布拉格反射半导体激光器DBR LD实现高分辨力亚纳米精度测量的方案。理论计算表明,DBR LD的波长连续调制深度比F-P腔LD高一个量级。指出邮于DBR LD的特殊结构可通过简单的反馈回路稳定输出光功率,有效地避免了光强波动对测量精度提高的限制。     

C-band external-cavity wavelength-tunable laser based on a liquid-crystal deflector

A novel C-band external-cavity wavelength-tunable laser is proposed. The laser consists of a semiconductor gain chip, a collimating lens, a fixed etalon, a liquid-crystal deflector and a diffraction grating in a Littrow configuration. The lasing wavelength of this tunable external-cavity laser can be tuned to 19 wavelength channels of 100 GHz spacing. All channels are within 2.5 GHz of the ITU grids with a side-mode suppression ratio of approximately 35 dB over the whole range.     

Increasing the output of a Littman-type laser by use of an intracavity Faraday rotator

We present a method of external-cavity diode-laser grating stabilization that combines the high output power of the Littrow design with the fixed output pointing of the Littman-Metcalf design. Our new approach utilizes a Faraday-effect optical isolator inside the external cavity. Experimental testing and a model that describes the tuning range and optimal tuning parameters of the laser are described. Preliminary testing of this design has resulted in a short-term linewidth of 360 KHz and a side-mode suppression of 37 dB. The laser tunes mode hop free over 7 GHz, and we predict that much larger tuning ranges are possible.     

Synchronous tuning of extended cavity diode lasers: the case for an optimum pivot point

Diode lasers can be tuned by simultaneous rotation and translation of an external grating. This can be achieved by rotating the grating about a displaced pivot point. We derive the tuning range as a function of pivot point position for various extended cavity geometries. In each case, the geometric problem reduces to the solution of a quadratic equation. For near-infrared wavelengths, placement of the pivot is relatively noncritical for tuning ranges of the order of 10 GHz, but requires millimeter accuracy for a tuning range >100 GHz.     

Technique for continuous tuning of optical fiber lasers

For the first time to the authors' knowledge, we have demonstrated how thermally controlled overcoupled fused fiber couplers and fiber loop mirrors based on these couplers can be used as broadband tuning elements in a fiber laser cavity. No bulk optical elements play any role in this technique. Temperature tuning the coupler results in a shift in the coupling ratio or in the effective output coupler transmission. For a fixed pump source, and for a given laser cavity, this shift causes the lasing wavelength to shift. We have continuously tuned an Er silica fiber laser in this manner over the range of 1527-1570 nm in a ring configuration, and, using a fiber loop mirror with these couplers in a linear Tm silica fiber laser cavity, we have achieved more than 50-nm broadband tuning over the range of 1850-1910 nm. The tuning range and the sensitivity to temperature depend on the degree of overcoupling of the loop mirror coupler.     

A developed optical-feedback cavity ring-down spectrometer and its application

A developed spectrometer based on optical-feedback cavity ring-down spectroscopy (OF-CRDS) has been demonstrated with a distributed feedback laser diode and a V-shaped glass ceramic cavity. The laser is coupled to the V-shaped cavity, which creates an absorption path length greater than 2.8 km, and resonance between the laser frequency and the cavity modes is realized by modulating the cavity length instead of tuning the laser wavelength to obtain a higher resolution. A noise-equivalent absorption coefficient of ~2.6 × 10(-8) cm(-1)Hz(-1/2) (1σ) is determined with spectral resolution of ~0.003 cm(-1) and spectral range of 1.2 cm(-1). As an application example, the absorption spectrum measurement of water vapor in the spectral range of 6590.3~6591.5 cm(-1) is demonstrated with this spectrometer.     

Unidirectional single-mode Nd:YAG laser with a planar semimonolithic ring cavity

Unidirectional single-mode operation of a diode-pumped Nd:YAG laser with a planar semimonolithic ring cavity has been demonstrated at 1064 nm. The semimonolithic cavity consists of a laser active medium placed in a magnetic field, a crystal quartz plate, and an output coupling mirror, which form an optical diode by acting as a Faraday rotator, a reciprocal polarization rotator, and a partial polarizer, respectively. A single-mode output power of 155 mW and a slope efficiency of 17% were obtained with a 1.2-W diode laser at 809 nm. A laser linewidth of less than 100 kHz is inferred from a beat note frequency spectrum between two identical laser systems and continuous tuning to greater than 2 GHz was observed.     

Frequency locking of tunable diode lasers to a rubidium-stabilized ring-cavity resonator

We demonstrate a technique for locking the frequency of a tunable diode laser to a ring-cavity resonator. The resonator is stabilized to a diode laser that is in turn locked to an atomic transition in rubidium, thus giving it absolute frequency calibration. The principal advantage of the ring-cavity design is that there is no feedback destabilization of the laser. The cavity has a free-spectral range of 1.3 GHz and Q of approximately 35, which provides robust locking of the laser. The locked laser is able to track large scans of the cavity.     

Fringe pattern analysis with a parametric method for measurement of absolute distance by a frequency-modulated continuous optical wave technique

Interferometry associated with an external cavity laser of long coherence length and broad wavelength tuning range shows promising features for use in measurement of absolute distance. As far as we know, the processing of the interferometric signals has until now been performed by Fourier analysis or fringe counting. Here we report on the use of an autoregressive model to determine fringe pattern frequencies. This concept was applied to an interferometric device fed by a continuously tunable external-cavity laser diode operating at a central wavelength near 1.5 microm. A standard uncertainty of 4 x 10(-5) without averaging at a distance of 4.7 m was obtained.     

Wide-bandwidth mode-hop-free tuning of extended-cavity GaN diode lasers

We present a new approach for extended-cavity diode-laser tuning to achieve wide mode-hop-free tuning ranges. By using a multiple piezoactuated grating mount, the cavity length and grating angle in the laser can be adjusted independently, allowing mode-hop-free tuning without the need for a mechanically optimized pivot-point mount. Furthermore, synchronized diode injection-current tuning allows diode lasers without antireflection coatings to be employed. In combination these two techniques make the construction of a cheap, efficient, and easily optimized extended-cavity diode laser possible. A theoretical analysis is presented for optimal control of piezoactuator displacements and injection current to achieve the widest possible mode-hop-free tuning ranges, and a comparison is made with measurements. The scheme is demonstrated for blue and violet GaN lasers operating at approximately 450 nm and approximately 410 nm, for which continuous tuning ranges exceeding 90 GHz have been achieved. Examples of applications of these lasers are also given.     

Diode laser mode selection using a long external cavity

We describe a simple and practical scheme to control the operation of a diode laser in a specific longitudinal mode through the use of a long external cavity. We show that for a fixed laser current and temperature, the laser can selectively oscillate in several different modes by controlling the external-cavity length. With a simultaneous sweep of the laser current and the cavity length, the laser frequency can be continuously scanned by more than 10 GHz. This laser system has been used successfully in a laser cooling experiment of neutral atoms.     

Accurate frequency-tuning mechanism from a wedge prism in a single-mode tunable laser

We present a new, accurate frequency-tuning mechanism in a grazing-incidence cavity for singlelongitudinal-mode oscillation in a tunable laser. A wedge prism with a small apex angle is inserted between the tuning mirror and the grating for accurate frequency tuning. In this configuration, the laser frequency can be tuned precisely by rotation of the wedge instead of the tuning mirror. This fine-tuning mechanism offers the ability to tune the frequency over more than 400 GHz without mode hop and with a tuning accuracy that is 2 orders of magnitude larger than that of conventional tuning methods.     

Multifrequency erbium-doped fiber laser operating at room temperature

A method to realize room-temperature operation of a multifrequency Er-doped fiber laser with low-frequency shift feedback placed within a linear laser cavity is theoretically proposed and experimentally demonstrated. Simultaneous multiwavelength lasing with 0.5 nm wavelength spacing is experimentally demonstrated by applying a sinusoidal signal of 10 kHz to a fiber phase modulator inserted within the linear cavity to prevent single wavelength steady-state oscillation. In the linear cavity, an all-polarization-maintaining fiber Sagnac loop is used as a periodic filter, and a single-mode fiber loop with a polarization controller is used as a partial reflector and also as an output port.     

Wavelength tuning and spectral properties of distributed feedback diode lasers with a short external optical cavity

We report on the wavelength tuning and spectral properties of distributed feedback (DFB) diode lasers operated with a plane external cavity (XC) mirror positioned as close as possible to the diode-laser front facet. These lasers generate single-frequency near IR radiation at wavelengths of 1392, 1580, 1602, and 1653 nm. A piezoelectric variation of the XC length provided continuous single-frequency tuning to as high as 19 GHz. A further benefit of XC DFB lasers is a residual amplitude modulation per gigahertz tuning of less than 10(-3). The XC feedback also suppresses residual side-mode oscillations to less than 60 dB. The laser's total intensity noise is close to the shot noise limit. The laser linewidth (measured in a beat note experiment) is less than 90 kHz within an acquisition time of 40 ms. The advantageous properties of XC DFB lasers for molecular spectroscopy are demonstrated by recording R(3) 2nu(3) overtone spectra of methane by single-scan single-pass absorption or frequency-modulation spectroscopy.     

Deceleration of a calcium atomic beam with a frequency-doubled diode laser

A calcium atomic beam has been decelerated by a single extended-cavity diode laser, frequency doubled to 423 nm. A potassium niobate crystal is placed in an external power buildup cavity, and the second-harmonic laser beam, counterpropagating with the atomic beam, is tuned into resonance with the strong (1)S(0)-(1)P(1) transition of calcium. For input power of 78 mW at 846 nm, we generate 22 mW at 423 nm after correction for the reflectivity of our cavity output coupler. To keep the atoms always in resonance during the deceleration process, the Zeeman tuning technique was used.     

Synchronous acousto-optic tuning of free-space external-cavity lasers

Mechanically tuned external-cavity lasers in which the cavity and the mode filter are tuned synchronously are well known and commercially available. The synchronicity allows phase-continuous tuning to be obtained. Acousto-optic tuning has potential advantages in terms of speed, stability, and precision, but a method for synchronous tuning is required. We propose such a method and describe its experimental verification.     

Diffractive optical element used in an external feedback configuration to tune the wavelength of uncoated Fabry-Pérot diode lasers

Abstract

A diffractive optical element (DOE) has been designed, fabricated and used in an external feedback configuration to set the wavelength of operation of uncoated Fabry-Pérot diode lasers. The DOE was designed to replace the conventional elements of an external feedback system, which are a collimating lens and grating in the Littrow configuration. The goal was to simplify use of the external cavity laser by replacing the lens and grating with a single optical element while maintaining the performance that is achieved with a grating and lens. Four DOEs were fabricated with two different focal lengths and two different reflectivities. DOE external cavity lasers were tested for maximum tunable range and stability of the wavelength of operation and compared with an external cavity laser based on a lens and grating in the Littrow configuration. A 40 nm tuning range was achieved with the DOE external cavity laser and this is comparable with the tuning range of the external cavity lasers based on a grating and collimating lens in the Littrow configuration.