Mode-hop-free electro-optically tuned diode laser

A tunable external-cavity Littrow diode laser, with an intracavity electro-optical crystal, which combines a large mode-hop-free tuning range with high tuning speeds and high tuning accuracy, has been designed. A mode-hop-free single-mode tuning range of 50 GHz at 793 nm with an output power of 60 mW and tuning speeds of 1.5 GHz/micros is demonstrated. The instantaneous linewidth is smaller than 300 kHz. Generally mode-hop-free tuning is obtained when the elongation of the cavity is proportional to the cavity length measured anywhere across the beam. This elongation is achieved with an intracavity crystal in which the thickness, and thus the electric field inside it, varies across the beam.     

1 W tunable near diffraction limited light from a broad area laser diode in an external cavity with a line width of 1.7 MHz

A novel unstable external cavity for a broad area laser diode is presented. The cavity is based on a V-shaped setup that improves the slow axis beam quality by coupling the internal modes of a gain guided laser diode. The novelty here is the compact unstable resonator design without lenses in direction of the slow axis. For frequency stabilisation and to narrow the line width of the laser diode emission a diffraction grating in a Littrow configuration is used. With this setup up to 1 W of near diffraction limited light with a beam quality of M² ? 1.3 and a line width of 1.7 MHz could be achieved. The external cavity laser was tunable over a range of 35 nm (FWHM) around the center wavelength of 976 nm.     

Widely tunable diffraction limited 1000 mW external cavity diode laser in Littman/Metcalf configuration for cavity ring-down spectroscopy

We report on recent progress on external cavity diode lasers (ECDL) using a new concept of a Littman/Metcalf configuration. Within this concept one facet of the diode laser chip is used for coupling to a high quality Littman/Metcalf resonator whereas the other side of the diode laser chip emits the output beam. The alignment of the external resonator is independent from the alignment of the output beam and there is no need for any compromise in the alignment. This results in an improved behavior of the external resonator with the benefit of a drastic increase in power and single mode tuning.We investigated this light source for high resolution spectroscopy in the field of cw-cavity ring-down spectroscopy (CRDS). The monitoring of environmental and medical gases from vehicles or human breath requires a suitable radiation source in the mid-infrared (MIR) between 3 and 5 μm that is frequency stable and can be widely tuned. Since this wavelength cannot be reached via direct emitting room temperature semiconductor lasers, additional techniques like difference frequency generation (DFG) are essential. Tunable difference frequency generation relies on high power, small linewidth, fast tunable, robust laser diode sources with excellent beam quality.With our new compact, alignment-insensitive and robust ECDL concept, we achieved an output power of 1000 mW and an almost Gaussian shaped beam quality (M²<1.2). The coupling efficiency for optical waveguides as well as single mode fibers exceeds 70%. The wavelength is widely tunable within the tuning range of 20 nm via remote control. This laser system operates longitudinally in single mode with a mode-hop free tuning range of more than 150 GHz without current compensation and a side-mode-suppression better than 50 dB. This concept is currently realized within the wavelength regime between 750 and 1080 nm.Our high powered Littman/Metcalf laser system was part of a MIR-light source which utilizes DFG in periodically poled lithium niobate (PPLN) crystals. At the wavelength of 3.3 μm we were able to achieve a high-resolution absorption spectrum of water with four different isotoplogues of H₂O components. This application clearly demonstrates the suitability of this laser for high-precision measurements. PACS 07.57.Ty; 42.55.Px; 42.62.Fi     

Fast mode-hop-free acousto-optically tuned laser with a simple laser diode

A mode-hop-free tunable external-cavity Littrow diode laser with intracavity acousto-optic modulators (AOMs) has been built. The modes of the red laser diode without a special antireflection coating are shifted by varying the injection current. The external resonator modes and the grating selectivity are independently electrically alterable by two AOMs. Thus, a tuning of the external resonator over up to 1900 GHz is possible. A precise computer control of laser diode and AOMs allowed a single-mode tuning of the whole laser with a tuning range of 225 GHz in 250 s. Additionally, we demonstrated fast tuning over 90 GHz in 190 micros and a repetition rate of 2.5 kHz.     

Continuously tunable external-cavity diode laser with a double-grating arrangement

A new external-cavity diode laser system based on an extended version of the Littman configuration is investigated. In our laser system the tuning mirror is replaced with a second grating mounted at the Littrow angle. This double-grating design provides a smaller passive bandwidth than the grating-mirror configuration, which is used to compensate for the large angle-of-incidence-dependent losses of the grazing-incidence grating. This results in a broader continuous tuning range with an improved mode stability. Because two laser diodes, emitting at 820 and 775 nm, respectively, are used, the external-cavity laser is continuously tunable without mode hops across 35 nm at 820 nm and 27 nm at 775 nm. Such a laser was used to measure the absorption lines of the oxygen A band around 762 nm. The complete R - and P -rotational branches of the b summation operator(g)(1) (+)(nu(??)=0)?X (3) summation operator(g)(-)(nu (?)=0) transition were recorded in a single-wavelength scan.     

Grating feedback in a 810 nm broad-area diode laser

Narrow linewidth, single spectral mode operation has been obtained in a high power, 810 nm broad-area diode laser in an extended cavity configuration with a grating as external reflector (grating feedback). For stable operation it was necessary to misalign the feedback slightly in the plane of the laser junction. Characteristics of the thus obtained laser system are a linewidth below 5 MHz, an output intensity of about 50% of the free running power, a large-scale tuning range of 15 nm and continuous scanning over 4 GHz. In the spatial domain, the laser remains multimode and astigmatic. To show the practical applicability of this system, saturated absorption of a krypton line is demonstrated.     

Tunable, μs-pulsed ytterbium fiber laser system with a linewidth below 2.7 GHz

We report on a widely tunable, pulsed laser system with narrow spectral linewidth based on a continuous wave ytterbium fiber oscillator, a pulse shaper and a power amplifier stage. The system is tunable from 1055 nm to 1085 nm and provides a maximum pulse energy of 155 μJ with a pulse duration of 1-5 μs. The linewidth is less than 2.7 GHz over the whole tuning range.     

High-power multiple-frequency narrow-linewidth laser source based on a semiconductor tapered amplifier

The output of two grating-stabilized external-cavity diode lasers was injected into a semiconductor tapered amplif ier in a master oscillator-power amplif ier (MOPA) configuration. At a wavelength of 671 nm this configuration produced 210 mW of power in a diffraction-limited mode with two frequency components of narrow linewidth. The frequency difference delta was varied from 20 MHz to 12 GHz, while the power ratio of the two components was freely adjustable. For delta < 2 GHz additional frequency sidebands appear in the output of the MOPA. This configuration is a f lexible and simple high-power cw laser source for light with multiple narrow-linewidth frequency components.     

Development of narrow linewidth, micro-integrated extended cavity diode lasers for quantum optics experiments in space

We present a micro-integrated extended cavity diode laser module for experiments on rubidium Bose–Einstein condensates and atom interferometry at 780.24 nm onboard a sounding rocket. The micro-integration concept is optimized for space application. The laser chip, micro-lenses, a volume holographic Bragg grating, micro-temperature sensors and a micro-thermoelectric cooler are integrated on an aluminium nitride ceramic micro-optical bench with a foot print of only 50 × 10 mm². Moveable parts are omitted to allow for a very compact and robust design. The laser module provides an output power of more than 120 mW at a short term (170 μs) linewidth of 54 kHz, both full-width-at-half-maximum. The laser can be coarsely tuned by 44 GHz with a continuous tuning range of 31 GHz. The micro-integration technology presented here can be transferred to other wavelengths.     

Vertically emitting, dye-doped polymer laser in the green (λ ∼ 536 nm) with a second order distributed feedback grating fabricated by replica molding

Lasing in the green from a distributed feedback (DFB) structure, based upon a second order grating fabricated by replica molding in a dye-doped UV curable polymer, has been demonstrated. For a Bragg grating having a periodicity and depth of 360 ± 2 nm and 78 ± 5 nm, respectively, a coumarin 540-polymer laser operates at 535.6 nm, which is in agreement with calculations of the photonic band diagram for the structure. The fabricated laser exhibits a linewidth of 0.15 nm, a threshold pump fluence of ∼0.7 mJ cm⁻² at 355 nm, and a slope efficiency of ∼14%. Incorporation of the dye gain medium into a one- (or two-) dimensional photonic crystal and fabrication of the grating by replica molding at room temperature provides an inexpensive approach to fabricating polymer-based DFB lasers on flexible substrates of large area.     

An external cavity diode laser using a volume holographic grating

This study presents an external cavity diode laser (ECDL) system, utilizing a volume holographic grating (VHG) and a microfabricated silicon flexure as the VHG holder. The laser design is aimed for easy assembly, controllability, and better stability of the laser cavity. The laser frequency was stabilized to a D₂ transition of rubidium at 780.247 nm, with a mode-hop-free tuning range of 16 GHz and 9.6 GHz with and without feed-forward on the diode injection current. The measured linewidth was 850 kHz in 500 s, qualified for laser cooling experiments.     

A tunable diode-laser spectrometer for the MIR region near 7.2 μm applying difference-frequency generation in AgGaSe2

2 and two diode lasers as pump sources are presented. A single-mode Fabry–Pérot-type tunable diode laser (TDL) and an external-cavity diode laser (ECL) were combined to generate radiation in the mid-infrared region near 7.2 μm. With a TDL at a wavelength of approximately 1290 nm and an ECL emitting between 1504 and 1589 nm it was possible to carry out spectroscopic experiments concerning SO₂ at five different phasematching points between 1350 and 1400 cm^-1 by fixing the wavelength of one pump laser and tuning the wavelength of the other. With an input power of 8 mW for the single-mode Fabry–Pérot-type diode laser and 6 mW for the external-cavity laser an output power of about 10 nW was generated. Using the tuning capabilities of the external-cavity laser a spectral region up to 5 cm^-1 could be covered within one scan. Measurements of SO₂ absorption lines at low pressure demonstrate the high-resolution features of the spectrometer. Moreover, these data provide new direct experimental phasematching data for the rarely investigated spectral region at 7.2 μm. Received: 27 October 1997/Revised version: 8 May 1998     

Simple high-coherence rapidly tunable external-cavity diode laser

We describe a simple electro-optically activated external-cavity diode laser designed to provide high-speed, high-coherence tuning over gigahertz frequency ranges. Tuning as fast as 23 GHz/ micros is demonstrated. Coherence measurements indicate transform-limited quiescent laser linewidth in observation windows as wide as ~100 micros and transform-limited chirps. A self-heterodyne, cross-correlation-based coherence diagnostic is developed for characterizing phase coherence during high-speed chirps.     

Sub-5-pm linewidth, 130-nm-tuning of a coupled-cavity Ti:sapphire oscillator via volume Bragg grating-based feedback

A novel coupled-cavity Ti:sapphire oscillator architecture featuring a volume Bragg grating as a feedback element is presented. The oscillator provides continuous wave lasing within a spectral linewidth as narrow as 5 pm. The output can be wavelength-tuned over an ultrabroad spectral range of 130 nm, extending from 714 to 842 nm. This unique combination of narrow spectral linewidth and wide tuning range makes the laser suitable for applications such as sensing and Raman and absorption spectroscopy. The laser also displays ideal TEM₀₀ mode operation throughout its tuning range with output powers beyond 300 mW. Detailed studies of the cw lasing dynamics across the wide tuning range are described. The general architecture of this design can be implemented for high resolution tuning across the broad spectral emission bands of other solid state lasers with single mode operation.     

Ti:sapphire laser intracavity difference-frequency generation of 30 mW cw radiation around 4.5 μm

A cw mid-IR coherent source based on difference-frequency generation is designed and characterized. For mid-IR generation, a periodically poled MgO:LiNbO(3) crystal is placed inside a compact Ti:sapphire laser cavity. This provides high-power pump radiation for the nonlinear process. Optical injection by an external-cavity diode laser ensures single-frequency operation of the Ti:sapphire laser, while signal radiation is provided by a fiber-amplified Nd:YAG laser. Mid-IR radiation can be generated with 3850-4540 nm tuning range, narrow linewidth, Cs-standard traceability, and TEM(00) spatial mode. 30 mW power is obtained at 4510 nm.     

用于铷蒸气激光泵浦的窄线宽阵列半导体激光器

在激光二极管LD泵浦铷蒸气激光器中,窄线宽半导体激光是实现铷蒸气激光高效率输出的关键技术之一。基于体布拉格光栅（VBG）外腔技术,实现了40 W功率0.14 nm线宽的780 nm激光输出。采用半导体制冷片（TEC）控制VBG温度,使得该激光器空气中波长可从779.35 nm调谐至780.10 nm,可用于铷蒸气激光的高效泵浦。     

On enhanced tuning capabilities of external cavity lasers using acousto-optic modulators

For an enhanced tuning of an external-cavity Littrow diode laser at 670 nm a new approach based on the frequency controlling of intra-cavity acousto-optic modulators (AOM) has been developed and tested. Using a method called “mode shaking” a nearly continuously laser tuning over a frequency range of 186 GHz (0.279 nm) within a time interval of 20 ms has been achieved. A further method called “far laser frequency jump” allows frequency changes of up to 1080 GHz (1.6 nm) between two spectral regions of the diode laser emission range within about 50 μs.     

Narrow-linewidth widely tunable hybrid Q-switched double-clad fiber laser

A laser-diode-pumped Yb-doped double-clad fiber laser operating in a hybrid Q-switched regime has been demonstrated. With pulsed pump light and stimulated Brillouin scattering of the gain fiber as the Q-switching mechanisms, the laser generated nanosecond pulses with a stable repetition rate. A single-pulse energy of as much as 143.1 microJ with a peak power of 28.6 kW was obtained. Use of an external-cavity diffraction grating in the Littman configuration permitted tuning of the laser wavelength over a 15.7-THz range from 1080 to 1140 nm, and a linewidth of 0.04 nm over the whole tuning range was achieved.     

Wavelength tunability of L-band fiber ring lasers using mechanically induced long-period fiber gratings

We report on oscillation wavelength control in erbium-doped fiber ring lasers by adjusting the period of a mechanically induced long-period fiber grating (LPFG) inserted into the fiber ring resonator. Pump light is provided by a 974 nm laser diode (LD), the emission of which is coupled into the fiber ring resonator through a wavelength-division multiplexing coupler. Laser oscillation occurs with a threshold pump LD current of 40 mA, corresponding to a threshold pump power of 5 mW. When a periodic pressure of 0.81 N/mm is applied to form the LPFG, the fiber ring laser exhibits the tunable range of 40.9 nm, i.e., from 1563.1 to 1604 nm, by changing the grating period.