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.     

Gain flattening coatings for improved spectral performance of asymmetric multiple quantum well lasers

A gain flattening coating was designed and fabricated to enhance the wavelength tuning for asymmetric multiple quantum well (AMQW) lasers. After coating, a nonlasing gap in the middle range of the lasing wavelengths, which might exist for AMQW lasers that are operated without an external cavity, was overcome and the total lasing range was increased. With the coating, the tuning range of an AMQW laser, as measured without an external cavity, was increased to 85?nm from 70?nm.     

Sensitive absorption spectroscopy by use of an asymmetric multiple-quantum-well diode laser in an external cavity

We have developed a broadly tunable diode laser system by employing custom-designed asymmetric multiple-quantum-well (AMQW) InGaAsP lasers in an external cavity configuration. Feedback is provided by a diffractive optical element with high coupling efficiency and wavelength selectivity, allowing for single-mode tuning of the output wavelength by varying the external cavity length. This tunable laser system was used experimentally to perform absorption spectroscopy on weak CO(2) lines over a broad wavelength region in the near infrared. An experimental tuning range of 80 nm has been observed for a laser cavity length of 600 mum, which is double the tuning range found with conventional, uncoated quantum-well lasers. We achieved a detection sensitivity of 5 x 10(-6) at 95% confidence over the wavelength range of 1.54-1.62 mum by employing a second-harmonic detection technique. The theoretical predictions of a broad gain profile from an ambipolar rate equation model are found to correspond to the experimentally observed increased tunability of the uncoated AMQW lasers.     

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.     

Widely Tunable Continuous-Wave CR(3+):LiSrAlF(6) Ring Laser from 800 to 936 nm

We report on a widely tunable, diode-pumped, continuous-wave, single-frequency Cr(3+):LiSrAlF(6) laser with a novel double-ring cavity configuration. The optical diode and the tuning element were located in the ring subcavity to decrease their insertion loss. Using an 1800-lines/mm grating as the tuning element, we obtained a laser emission of 20-mW output power at 870 nm and a tuning range of from 800 to 936 nm. When we used a 1200-lines/mm grating with a higher diffraction efficiency, the laser had a slightly wider tuning range of from 800 to 938 nm. However, the output power decreased at this tuning range.     

Sub-micrometre distance measurements with a broadly tunable short-external-cavity InGaAsP/InP diode laser

Measurement of distance using a custom-designed, broadly tunable InGaAsP/lnP short-external-cavity diode laser is described. A tuning range of over 100 nm was achieved with the custom-designed laser in a diffractive optical element short external cavity. This tuning range made it possible to achieve a sub-micrometre resolution in measurement of distance with a single laser source for an interferometer. A non-linear, least squares fitting method was used to extract the displacement from the raw data. This fitting method showed a potential for extraction of accurate displacement in the presence of noise.     

Wavelength-shift speckle interferometry for absolute profilometry using a mode-hop free external cavity diode laser

Abstract

In this paper we present an absolute surface topography measurement with a tuneable diode laser with an external cavity that has a tuning range of as much as 25 nm without mode hops and a very high tuning speed of less than 1 s for the whole range. With this laser, an absolute wavelength-shift speckle pattern interferometer was realized, capable of measuring optically smooth and rough surfaces. We briefly explain the principles of operation, and the importance of wavelength tuning without mode hops. We discuss the capabilities, possible measuring ranges and some results as well as calibration methods of wavelength-shift speckle profilometry.     

Relaxation of the single-mode emission conditions in extended-cavity semiconductor lasers with a self-organizing photorefractive filter

Commercial 1.55-microm extended-cavity semiconductor lasers provide single-mode operation that can be continuously tuned over a range larger than 100 nm without mode hopping. But such performance requires delicate factory adjustment and high mechanical stability of the external cavity. Furthermore, at high emission power the tuning range is limited to small values because of the annoying multimode operations that sometimes occur. We have shown that the alignment constraints can be relaxed by use of an intracavity photorefractive filter. Here we present new results obtained with a crystal with low absorption and high photorefractive gain. We demonstrate that, without inducing excessive additional loss, we can preserve single-mode emission at an output power higher than the maximum power obtained in the absence of a photorefractive crystal over the full tuning range of the laser.     

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.     

Tunable high-power narrow-spectrum external-cavity diode laser at 675 nm as a pump source for UV generation

High-power narrow-spectrum diode laser systems based on tapered gain media in an external cavity are demonstrated at 675 nm. Two 2 mm long amplifiers are used, one with a 500 μm long ridge-waveguide section (device A), the other with a 750 μm long ridge-waveguide section (device B). Laser system A based on device A is tunable from 663 to 684 nm with output power higher than 0.55 W in the tuning range; as high as 1.25 W output power is obtained at 675.34 nm. The emission spectral bandwidth is less than 0.05 nm throughout the tuning range, and the beam quality factor M(2) is 2.07 at an output power of 1.0 W. Laser system B based on device B is tunable from 666 to 685 nm. As high as 1.05 W output power is obtained around 675.67 nm. The emission spectral bandwidth is less than 0.07 nm throughout the tuning range, and the beam quality factor M(2) is 1.13 at an output power of 0.93 W. Laser system B is used as a pump source for the generation of 337.6 nm UV light by single-pass frequency doubling in a bismuth triborate (BIBO) crystal. An output power of 109 μW UV light, corresponding to a conversion efficiency of 0.026% W(-1), is attained.     

Innovative design of a tunable laser using liquid crystal tuning elements

An innovative non-mechanical and low power consumption tunable external cavity laser (ECL) using liquid crystal tuning elements is proposed. This contains a gain chip, a collimating lens, tuning elements and a partial-reflection mirror. The proposed tunable ECL can achieve both coarse tuning and fine tuning, and it is designed to lase at wavelength matching the International Telecommunication Union (ITU) channels, which is one of the important requirements in optical communication. The tuning elements include an ITU etalon, a liquid crystal Fabry–Pérot interferometer (LC-FPI) and a fine tuner. Only two parameters are required to tune the wavelength over the whole C-band, namely the voltage over the LC-FPI and the fine tuner. This high reliability cost-effective design proposes a theoretical tuning range of about 80?nm. The LC tuning elements including LC-FPI and fine tuner has been fabricated and tested.     

Extending the continuous tuning range of an external-cavity diode laser

The continuous tuning range of an external-cavity diode laser can be extended by making small corrections to the external-cavity length through an electronic feedback loop so that the cavity resonance condition is maintained as the laser wavelength is tuned. By maintaining the cavity resonance condition as the laser is tuned, the mode hops that typically limit the continuous tuning range of the external-cavity diode laser are eliminated. We present the design of a simple external-cavity diode laser based on the Littman-Metcalf external-cavity configuration that has a measured continuous tuning range of 1 GHz without an electronic feedback loop. To include the electronic feedback loop, a small sinusoidal signal is added to the drive current of the laser diode creating a small oscillation of the laser power. By comparing the phase of the modulated optical power with the phase of the sinusoidal drive signal using a lock-in amplifier, an error signal is created and used in an electronic feedback loop to control the external-cavity length. With electronic feedback, we find that the continuous tuning range can be extended to over 65 GHz. This occurs because the electronic feedback maintains the cavity resonance condition as the laser is tuned. An experimental demonstration of this extended tuning range is presented in which the external-cavity diode laser is tuned through an absorption feature of diatomic oxygen near 760 nm.     

Single-mode tunable Ti:sapphire laser over a wide frequency range

We report a design of a tunable Ti:sapphire laser capable of operating in the range between 700 and 800 nm. The continuous, single-mode tuning is achieved by a pseudoexternal cavity consisting of highly reflective mirrors and a diffraction grating. The advantages of this laser include low operational threshold, a simple configuration that involves only four optical elements, and fine-tuning capabilities. The single longitudinal mode of operation was demonstrated at wavelengths between 695 and 725 nm and was limited by the choice of end mirrors in the laser cavity.     

Broadband amplifier and high performance tunable laser with an extinction ratio of higher than 60 dB using bismuth oxide-based erbium-doped fiber

A Bi₂O₃-based erbium-doped fiber (Bi-EDF) ring laser with a 70?nm tunable range is demonstrated with a 49?cm long Bi-EDF in which tuning range can be extended to larger than 100?nm using an optical switch to alter the length of Bi-EDF in the laser cavity. With an extinction ratio of better than 60?dB throughout the entire tuning range, the measured FWHM of laser lines are measured to be 0.09?nm. In addition, the common amplification parameters are measured and studied in detail for various pumping configurations.     

Spectral linewidth narrowing and tunable two-color laser operation of two diode laser arrays

We propose and implement a common external cavity to narrow spectral linewidth of two broad-area laser diode arrays (LDAs) and align their center wavelengths. The locked center wavelength of two LDAs can be tuned in the range of ~10 nm by tuning the tilted angle of the diffraction grating. The output beams of two LDAs are spatially overlapped through the polarization beam splitter of the common external cavity, and the total output power equals the power of two LDAs. The center wavelength of each LDA can be independently tuned by shifting the corresponding fast-axis collimation lens. As a result, the high-power two-color LDA operation is demonstrated with the tunable wavelength difference of up to 2 nm (~1 THz).     

Tunable 6.8 W narrow bandwidth emission from a single-stripe continuous-wave broad-area laser diode in a simple external cavity

An antireflection-coated broad-area laser diode with an emitter size of 400 microm x 1 microm and a chip length of 1500 microm is operated in a simple external cavity. For wavelength stabilization and to narrow the bandwidth a diffraction grating in a Littrow configuration is used. At an injection current of 9 A up to 6.8 W of optical output power and a resulting slope efficiency of 0.8 W/A could be achieved. Further, the bandwidth could be narrowed to 100 pm (FWHM), and a tuning range of 40 nm around 976 nm was obtained.     

Tunable laser diode system for noninvasive blood glucose measurements

Optical sensing of glucose would allow more frequent monitoring and tighter glucose control for people with diabetes. The key to a successful optical noninvasive measurement of glucose is the collection of an optical spectrum with a very high signal-to-noise ratio in a spectral region with significant glucose absorption. Unfortunately, the optical throughput of skin is low due to absorption and scattering. To overcome these difficulties, we have developed a high-brightness tunable laser system for measurements in the 2.0-2.5 microm wavelength range. The system is based on a 2.3 microm wavelength, strained quantum-well laser diode incorporating GaInAsSb wells and AlGaAsSb barrier and cladding layers. Wavelength control is provided by coupling the laser diode to an external cavity that includes an acousto-optic tunable filter. Tuning ranges of greater than 110 nm have been obtained. Because the tunable filter has no moving parts, scans can be completed very quickly, typically in less than 10 ms. We describe the performance of the present laser system and avenues for extending the tuning range beyond 400 nm.     

环形倍频腔系统稳定性的分析

为了实现通过腔外倍频钛宝石可调谐激光(851．1m)而得到所需波长(425．6nm)的光。首先根据二次谐波产生理论讨论了针对利用LBO晶体倍频851．1nm光时有关晶体参数的选择，然后在此基础上利用矩阵光学的方法分析了环型外谐振倍频腔的几何参数，包括腔的尺寸，反射镜的曲率半径等对系统稳定性的影响及倍频光输出功率稳定性的影响，并作了数值计算和分析。     

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.     

Operation of a Ti:sapphire laser pumped by a 499-nm green laser

We report, for the first time, to our knowledge, the operation of a tunable Ti:sapphire laser pumped by a third-order Raman XeCl-H(2) laser system at 499 nm with a 60-ns pulse duration. The slope efficiency is 59% for this laser, producing pulses of 20-ns duration. The highest conversion-energy efficiency obtained is 41%, with an output energy of 1.2 mJ. The tuning range for a single set of cavity mirrors is 680-834 nm and is limited mainly by the mirror reflectivity. This study shows that a combined laser system based on a XeCl excimer laser can offer wavelength diversity.