Desarrollo de la Propuesta

Diode lasers are classified as solid state lasers, where the laser or lasing medium is usually made up of doped solid crystalline materials (e.g.; ruby, Nd-YAG, Nd-Glass, Nd-YLF, etc). Diode lasers, also called semiconductor lasers, are the smallest ever made lasers with an active medium of grain size crystal, which is usually cut in a rectangular shape with cleaved facets to work as the laser resonator. The other facets are destroyed by using different methods like etching, grounding, ion implantation....etc. These little tiny crystals are basically a combination of some doped semiconductor materials or alloys in a p-n junction. The frequency range of a diode laser is commonly determined by the exact composition of the semiconductor crystals, which is precisely selected and controlled in the manufacturing phase. The laser action takes place in these crystals when a voltage is applied on the p-n junction. Then holes and electrons are generated in the junction, thereby creating a population inversion within the junction. Electrons and holes then recombine and emit the

recombination energy as a laser radiation which covers so far the visible and infrared regions depending on the composition of the laser medium.

The diode laser spectrometer used in this work is the commercially available model (Mutek MSD 1100), consisting of the laser diodes, the cryostat and the optics. The diode lasers are

Fig. 4.1: Experimental setup of the tunable diode laser spectrometer system in our lab.

Ref. (125)

lead salt lasers from Laser Component and Aero Laser companies. The active medium is a combination of crystalline structure from (PbSe, PbTe, PbEu…etc). Lead salt diode lasers have a wavelength emission range between 3 and 15 µm or (3300-650) cm-1. These lasers provide a typical output power from 100µ W to 1 mW with a typical emission line width of 30-100MHz. Each diode laser has a quasi continuous spectral coverage over a region of 50-150 cm-1. More than 50 of these diodes are available along with the spectrometer in our lab, which cover a tunable wavelength range of 900-2800 cm-1. The rectangular and extremely small size (50-200) µm laser cavity, results in a highly divergent (20-40 degrees) beam which suffers from astigmatism

and elliptical beam profile. These drawbacks of the laser beam lead to inhomogeneous broadening of the gain profile which results in multimode laser radiation or emission across the frequency range of the diode laser. These modes are typically separated by 1 to 4 cm-1 and can be continuously tuned over a frequency range of 0.5 to 2 cm-1. Therefore, single mode operation of these lasers is limited to small regions and only possible in certain cases. In principle, all diode lasers have a similar overall performance, but each diode laser is a unique device with highly individual characteristics that depends on the composition of the semiconductor crystal and the applied current and temperature. Even diode lasers from the same crystal may have unique beam properties.

The sensitivity of the spectrometer is not limited by the1 f laser noise that shows up to frequencies of 100MHz, but through etalon structures in the signal. This can be caused by every pair of reflecting surfaces in the beam path which in our setup are for example the 12 cm diameter mirrors of the Herriott multi-pass-cell. But the pump vibrations are transferred to the mirrors of the cell as they are very heavy. These mirror vibrations destroy the phase coherence of the etalon signals, so that they are mostly damped and show up rarely. The distinct improvement of the signal-to-noise ratio is depicted exemplarily in (75).

The wavelength emission of diode lasers is a function of the diode temperature and the applied current. The coarse tuning is mainly done by varying the diode temperature, while the fine tuning is achieved by smooth changes of the applied current; i.e. continuous tuning over a small limited range or across a selected longitudinal mode. In coarse tuning, the temperature change causes a variation in the band energy gap and a modification of the cavity length of the diode laser due to the changing refractive index (n) of the semiconductor crystal. These changes lead to the so called mode jumping where different modes are generated to fit different cavity lengths, i.e. one mode is terminated and a second one is generated at another temperature to fit the new cavity length. The second tuning method of the diode laser is based on changing the applied current while the temperature being held fixed. This normally produces a small amount of Joule heating that causes a slight change in the diode temperature and leads to alteration of the refractive index (n). In this method the change of refractive index results in a negative tuning rate ∆ν ⁄∆I, while the

temperature increase yields a positive tuning rate. However, these changes shift the laser modes in the same direction as the band gap change, but at a slower rate, thus, providing a more precisely and controllable way of continuous tuning over limited ranges, i.e. single mode range. The very short time scale of current tuning ≤ 1µs compared with the time scale of the temperature tuning 5-30 seconds makes it more profitable or suitable to use for scanning the diode lasers over their frequency range. The typical tuning rates are:

Current tuning: ∆ν ⁄ ∆I = 0.2-3 GHz ⁄ mA Temperature tuning: ∆ν ⁄ ∆T = 10-100 MHz ⁄ mK

4.1.2

Cryostat

Lead salt diode lasers operate at cryogenic temperatures, i.e. < 80 K. A closed-cycle helium cooler from Leybold is used to cool the laser diodes down to 20 K; it also has a precise temperature control over the working range of the diode laser between 20 and 70 K. It is a long term, maintenance free system with a water-cooled compressor. The cryostat is mechanically isolated against the vibration of the helium cooler. Four different diode lasers can be accommodated and simultaneously cooled down in the cryostat chamber. The desired temperature of the diode laser is achieved by resistive heating, i.e. by changing the current through a heating coil plugged to the cold finger of the copper cold head which hosts the diode lasers. A special temperature controller with the required accuracy over the whole range (10-200K) was developed in our group, since such a controller was not commercially available; it utilizes a platinum resistor (Pt1000) as a temperature sensor which guarantees high stability, absolute accuracy (± 0.5 K), excellent reproducibility (0.05-0.1 K) and quick response (76). The actual control is achieved by using a precision analog PID (Potential-Integration- Differential) controller designed also in our group to produces an out put voltage which drives the heater current of the diode laser. The current can be adjusted between 0 and 900 mA with smallest step of 0.3 µA.

A set of compensated mirror optics consisting of two ellipsoidal mirrors with foci of 40mm and 140mm, one toroidal mirror with (f = 110mm) and some plane mirrors are

used to both select one of the four laser diodes in the cryostat and to collimate the diffraction broadened laser beam which exits the cryostat via a tilted CaF2 plane

window. The collimated output laser beam is then passed through a telescope arrangement (two mirrors with f = 60 cm and f = 10 cm) to reduce the beam diameter from 14 mm to 3mm. This is the optimal beam size required for coupling into the multi-pass cell located inside the vacuum chamber to exclusively probe the expansion zone of the slit nozzle. This extends a few cm vertical to the expansion direction, thereby increasing the signal to noise ratio. Purely reflective optical elements are used in the optical path of the laser beam in order to minimize the feedback to the diode lasers. No lenses are used in the optical setup as they act as a source of small back reflection in the laser cavity. The optimal laser beam is then guided to enter the vacuum chamber through a CaF2 window, where it is coupled in a Herriott multi-pass

cell. The cell arrangement consists of two spherical gold plated and identical concave mirrors separated by a distance of their radius of curvature; both mirrors have a diameter of 12 cm and a focal length of 50 mm. The design is made up to couple the optical beam into the cell through a hole in the first concave mirror. A correct alignment of the optical beam in the cell results in an elliptical beam spot pattern on both mirror surfaces, a maximum number of 40 spots can be achieved between the two mirrors before the beam emerges out of the same coupling hole as it entered. The number of spots usually determines the number of the beam reflections within the two mirrors and also specifies the optical absorption length in the cell which is between 80 and 160 cm for this design. This cell was built and integrated into the apparatus in context of a research Master degree project done in our research group (77). This design enables all reflections of the ellipse to be utilized, whereas in the old White cell design developed by König (70), only half of the ellipse could be used as shown in the fig. (4.2). The new cell has brought a further positive aspect for spectroscopy, apart from the increased number of passes and therefore the increased absorption length from 80 to 160 cm as shown in fig. (4.3) (77).

Fig. 4.2: Herriott cell design from König, Ref. (125)

The slit nozzle is usually aligned in the middle of the Herriot type cell to ensure that the molecular jet is generated 5 to 7mm perpendicular to the laser beam which crosses the narrow expansion zone at each path. As leaving the vacuum chamber, the laser beam is directed into a monochromator to separate and select the desired mode from the multimode emission of the diode laser. The monochromator employed in our diode laser spectrometer system is a 0.5 m Czerny Turner type (Mutek MDS1200) from Mutek Company with a frequency resolution of ~ 1 cm-1. A grating of 30 lines / mm with blaze wavelength of 25 µm

Fig. 4.3: New Herriott Cell design from Lehnig, Ref. (125)

is used in this monochromator which allows coverage of the whole pertinent wavelength range from ~ 800-3000 cm-1 by scanning over the different grating orders. Absolute wavelength calibration of the monochromator is done by using a He-Ne

laser. The absorption lines of CH4 monomer gas have been used to provide absolute

frequency calibration of the spectra with an accuracy of 0.001 cm-1. Spectral frequency calibration is achieved by deflecting a (~ 70%) fraction of the laser radiation using a ZnSe beam splitter and send it through a highly stable confocal etalon with a free spectral range of 0.01cm-1 (300 MHz). The etalon transmission is then used by the computer control program to determine the tuning rate with an accuracy of better than 1% and readjust the grating accordingly. The two portions of the beam are focussed onto HgCdTe-detectors; the signals are then amplified and detected by means of Stanford Research phase-sensitive lock-in amplifiers.

The reference frequency is generated by modulation of the diode laser current with a frequency of 7 kHz and amplitude between 0 and 1 mA, while the laser frequency is increased. The line width of the emission of a typical diode laser is between 50 and100 MHz. The modulation is thus adjusted in a way so that the spectral lines have an optimal intensity, without being significantly broadened due to the modulation

frequency. As a result of frequency modulation of the diode laser, one obtains a 1f derivative of the line profile, after demodulation in the lock-in amplifier. In order to avoid the difficulty of the frequency determination of the spectral lines by certain fluctuations of the central point of this derivative, one should take the second harmonic of the demodulated signal at 14 kHz. This procedure will differentiate the signal once again, so that the line frequency corresponds to the maximum of the line- shape once more. This new differentiation of the signal also suppresses background fluctuations with small gradients efficiently. The calculated line widths using the second harmonic, depending on the modulation, lie in the region of 30-100 MHz. Despite the fact that absorption spectroscopy is a relatively simple technique, sensitivities of as low as ∆I/I = 10-5

– 10-6 can be achieved (78, 79).

The spectrometer is controlled by means of a computer, programmed with LabVIEW

(71)

, which also enables, besides the spectroscopic measurements, a characterization of the laser diode by measurement of the mode chart.