Simple External Cavity Research Articles

785 nm grating-coupled external-cavity laser for shifted-excitation Raman difference spectroscopy

A 785 nm grating-coupled external-cavity (EC) laser was fabricated and applied as a light source for shifted-excitation Raman difference spectroscopy (SERDS) to extract Raman signal from the fluorescence background. A simple Littrow-type EC configuration with the grating lines parallel to the p–n junction was applied to narrow the linewidth of a commercially available high-power broad-area laser diode. The characteristics of the EC laser, including wavelength tuning range, output power, and emission spectra, were investigated in detail. A wavelength tuning range of more than 6 nm was realized at an injection current of 900 mA by simply changing the grating rotation angle. The output power after Raman probe coupling exceeded 40 mW and the spectral linewidth was narrowed significantly down to less than 0.1 nm from 0.9 nm (the free-running width). Then, five shifted emission lines were used for the Raman measurement with sesame oil as the target, whose Raman signal was overwhelmed in the fluorescence background. Due to the constraint relationship between the signal intensity and the spectral resolution, the best reproducibility of the Raman signal without fluorescence background was obtained when the excitation wavelength interval was fixed at around the linewidth of the Raman peak. In conclusion, the grating-coupled EC laser with flexible wavelength tunability, high output power, and narrow spectral linewidth was demonstrated to be an ideal light source for SERDS application.

Frequency stabilization at the sub-kilohertz level of an external cavity diode laser.

A simple external cavity diode laser (ECDL) in a Littrow configuration was established and actively frequency-stabilized by using a side-of-fringe stabilization technique. A wavelength tuning range of 60nm for the spectral range between 1000 and 1060nm was demonstrated by rotating the diffraction grating. A sub-kilohertz frequency stabilization of the ECDL was achieved. The linewidth of the laser was narrowed from 160kHz to 400Hz by laser frequency locking to a flank of a Fabry-Perot interferometer peak.

External cavity diode lasers with 5kHz linewidth and 200nm tuning range at 1.55μm and methods for linewidth measurement.

Two simple external cavity diode laser designs using fibre pigtailed gain chips are tested and their properties compared with a high end DBR fibre laser. These ECDLs demonstrate a FWHM linewidth as low as 5.2kHz with a fitted Lorentzian FWHM linewidth as low as 1.6kHz. Tuning ranges of 200nm covering 1420nm to 1620nm were demonstrated. To the best of our knowledge these are the narrowest linewidth and most broadly tunable external cavity diode lasers reported to date. The improvement in linewidth is attributed to greatly enhanced acoustic isolation allowed by using fiber coupled gain chips and by replacing kinematic mounts with a pair of rotatable wedges for cavity alignment which eliminates acoustic resonances. A detailed description and discussion of techniques used to characterize the frequency noise and linewidths of these lasers is provided.

Tunable 68 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 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.

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

Narrow-bandwidth high-power output of a laser diode array with a simple external cavity

Using a simple external cavity consisting of a grating and an etalon, we demonstrate bandwidth narrowing of a high power broad-area 3-stripe laser diode array. At a drive current up to 3.6 times the threshold current, the laser diode array can always be locked. An available output of 2.3 W is obtained with the bandwidth being narrowed from 2.2 nm to no more than 0.07 nm. By tilting the etalon, the narrowed wavelength can be tuned continuously in a range of 1.6 nm.

Fast spectral improvement in picosecond pulses generated from a DFB laser diode using a loosely coupled external cavity

A simple external cavity is constructed to provide adjustable feedback to a gain-switched distributed feedback laser diode. A 27-dB enhancement in the sidemode suppression ratio (SMSR) and a 0.3-nm reduction in the spectral linewidth have been observed. The improvement shows a saturation behavior when the feedback power reaches about -16 dBm. We report on the evolution and the dynamics of the spectral improvement. The steady state can be achieved as soon as four round-trip feedback cycles are completed. Dependences of the speed of improvement and the SMSR on the feedback pulse arrival time are also investigated. The optimal result can be obtained over a sensitive time window of about 20 ps.

Deriving Expression of Output Spectrum from Semiconductor Laser with Two Sub-cavities

An expression for the output spectrum of a semiconductor laser composed of two sub-cavities has been deduced within the frame of the mean-field approximation using the ray trace method. The expression is applicable to the simple external cavity semiconductor laser (ECLD), two-segment (absorptive bistable) semiconductor laser (ABLD), and other similar devices. For ABLDs and ECLDs, this expression has been used to investigate their characteristics. Especially, the effects of the interface reflection on the laser feature have been studied.

Polarization bistability in tuned, external-grating diode lasers

Spectral domains showing polarization-wavelength hysteresis regions up to 0.3 nm wide were observed when the emission frequency of a diode laser having a nearly polarization-independent confinement factor was tuned in a simple external cavity with grating. In these regions the laser emits in a stable single mode with either TE or TM polarization, depending on whether the region was entered from lower or higher wavelengths. We explain the observed phenomenon on the basis of nonlinear gain saturation between TE and TM polarization. >

620 mW near-diffraction-limited, 1.8 µm tapered laser

A tapered laser emitting near 1.8 µm wavelength is configured into a simple external cavity, and emits up to 1.44 W under quasi-CW operation, of which 620 mW is in a near-diffraction-limited pattern.

Measurement of photothermally induced distortion of thin films using an external cavity laser diode sensor

The thermal distortion of a 2.5 μm thick gold film on a glass plate, produced by a modulated Ar ion laser beam, was measured using a simple external cavity laser diode configuration. Experimental results agree well with calculations. We anticipate that the results of these measurements will help elucidate the photoinductive effect, the photothermally induced change in the inductance of a coil placed over the gold film.

Measurements of the linewidth of ridge-guide DFB lasers

Measurements of the linewidth of nominally 1.5 μm ridge-guide DFB lasers are presented. Values of ∼15 MHz at output powers of 10 mW have been measured. By using a simple external cavity the linewidth was reduced by approximately two orders of magnitude.

"Wide range cavity tuning for high-power X-band magnetrons"

Use of a simple external cavity to tune a magnetron over wide frequency ranges (10 per cent or greater) imposes performance limitations with respect to electronic efficiency, moding, cathode-temperature variation, and coupling change. This paper discusses the use of more than one external tuning cavity to improve magnetron performance. A demountable laboratory design which tunes over a range of 17 per cent is described, as well as a sealed-off tube having electronic efficiency, cathode temperature, and power output constant over a range of 10 per cent. The multiple-cavity method of tuning permits the use of much higher frequency ranges and power outputs.