External cavity lasers

In practical application, Raman sensors exclusively use frequency-stabilised laser sources to compensate for the low intensity of the Raman radiation. For Raman sensors, prevalently compact high-intensity external cavity laser diodes are used, operated in CW (continuous wave) mode. These diode lasers combine high intensity with the spectral stability required for Raman applications and are commercially available at various wavelengths. 

E. M. Shahverdiev, S. Sivaprakasam, and K. A. Shore. Parameter mismatches and perfect anticipating synchronization in bidirectionally coupled external cavity laser diodes. Phys. Rev. E, 66 017206, 2002. 

With such stable, narrow-line devices, it is straightforward to phase-lock two external cavity lasers. The experimental schematic is included in... 

Fig. 6a. The heterodyne signal is mixed with a local oscillator set to the heterodyne center frequency. The mixer output is then amplified in order to supply a feedback current that is proportional to the phase shift ofthe heterodyne signal. Note that because the tuning rate of the external cavity laser was 25 MHz/mA, the peak-to-peak variation in the feedback current over several seconds was less than 5 yA. Figure 6b shows the heterodyne signal under phase-locked conditions. The observed signal indicates that the phase-locked heterodyne linewidth is less than the 15 Hz resolution halfwidth of the spectrum analyzer.

Fig. 4. External cavity laser assembly. Laser mounts on... 

Fig. 5. Heterodyne signal of two free-running external cavity lasers.

Fig. 6. b) Heterodyne signal of two phase-locked external cavity lasers. Linear vertical scale 10 Hz/div, 10 Hz resolution band-... 

Tanaka, T., Takahashi, H., Oguma, M., Hashimoto, T., Hibino, Y., Yamada, Y, Itaya, Y, Albert, J., and Hill, K. O. (1996). Integrated external cavity laser composed of spot-size converted LD and uv written grating in silica waveguide on Si, Electron. Lett. 32, 1202— 1203. 

R. Maulini, M. Beck, J. Faist, E. Gini, Broadband tuning of external cavity bound-to-continuum quantum-cascade lasers. AppZ. Phys. Lett, 84 (10) 1659 (2004). 

For example, a diode-laser sensor was applied to measure CO and CO2 concentrations in combustion gases using fast extraction-sampling techniques [8]. The sensor was based on an external cavity diode laser (ECDL) operating over the spectral region 6321-6680 cm which includes the i -branch of the CO band, the i -branch of the 2i/i + 2v2 + v, CO2 band, and selected lines of the... 

The entangled state, viz. [... 0 Ckstimulated emission to generate a photon state coherent to an external photon state by using an appropriate cavity. Lasers for instance are based on such processes. 

Figure 7.7. External cavity diode laser, such as the Spectra Diode Labs 8530. All components are contained in a 3 x 4 x 10 in. case, and the output power is 300 mW.

ECDL External Cavity Diode Laser OI Optical Isolator L Lens AOS Acousto-Optic Switch PZT Piezo-Electric actuator Me Cavity mirror APD Avalanche Photodiode PC Pressure Controller P Pump... 

T. Heil, I. Fischer, W. Elsafier, B. Krauskopf, K. Green, and A. Gavrielides. Delay dynamics of semiconductor lasers with short external cavities Bifurcation scenarios and mechanisms. Phys. Rev. E, 67 066214, 2003. 

As has been pointed out above, a laser basically consists of an active material and a resonator. The latter enables the build-up of certain resonant modes and essentially determines the lasing characteristics. In most conventional devices, the optical feedback is provided by an external cavity with two end mirrors forming the resonator. With the advent of polymers as active materials, various new feedback structures were invented. Initially, a microcavity resonator device of the type shown schematically in Fig. 6.13 a was employed [48]. 

Example 1.6 For a resonator length of r/ = 8 cm, which is typical of a diode laser with an external cavity, and X = 800 nm, the integer q becomes q = 2x 10. For a Doppler width of A = 1 GHz, a length change of Ad = 2 pm is sufficient to shift the laser frequency periodically over the absorption profile. 

External passive resonators may become advantageous when the absorption cell cannot be placed directly inside the active laser resonator. However, there also exist some drawbacks the cavity length has to be changed synchronously with the tunable-laser wavelength in order to keep the external cavity always in resonance. Furthermore, one has to take care to prevent optical feedback from the passive to... 

In order to achieve more accurate values for the ratio of electron mass to proton mass from which the fine-structure constant can be derived, a frequency comb in the infrared region was developed which can be used to measure vibrational-rotational transitions in HD+-ions. The frequency distance q between the modes of the comb, which equals the repetition rate of the femto-second pulses, is stabilized on the resonance frequency of a cryogenic ultra-stable sapphire resonator. For the measurement of the transitions in the HD+-ion diode lasers with external cavity and a grating for achieving single mode operation are used. Their frequency is stabilized onto the centre of the molecular transition and is then compared with the adjacent mode of the frequency comb. 

Both nondispersive infrared spectroscopy and cavity ring down spectroscopy (employing a near-infrared external cavity diode laser) can be used to determine the ratio in the CO2 of expired... 

Applications So far, intracavity laser spectroscopy has been applied primarily to the detection of absorption spectra of gaseous impurities such as NH3 and CH4 in the near-infrared region using a tunable broadband laser. Special DLs designed with an external cavity have also been investigated recently for this purpose. CRS has been applied successfully to trace element detection using the ICP as the atomization system. The detection limits observed are at sub-parts per billion level (e.g., 0.3 ng ml for lead) and comparable to the detection limits achieved with ICP-MS. 

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