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Emission and Absorption

Topics:
Black Body Radiation
Cavity Resonator
Einstein Coefficients
Lifetime of Excited States
Diodelaser
Nd:YAG Crystal
Interference Filter

This series of experiments investigates the absorption of diode laser radiation by the Nd:YAG. The ability to tune the wavelength by varying the temperature of the diode is exploited to examine the absorption spectrum. The resulting fluorescence as well as the four - level lasing system of Nd:YAG is investigated. The most common technique of generating a population inversion in solids, excluding semiconductor materials, is the process of optical pumping. The resulting pump efficiency depends on the choice of proper absorption and emission transitions. As an important example, the following set-up for optical pumping of Nd:YAG with a laser diode is introduced. In the experiment, the relevant pump and laser transitions are investigated and characterised. In the second step, the emission generated by the pumping process will be investigated and the life time of the initial energy level of the laser cycle will be measured.
As a pump light source, a 30 mW laser diode with a thermoelectric cooler is used. The diode laser radiation is collimated by means of microscope objective. A Nd:YAG rod is applied as the absorbing and emitting probe material. Collimating optics in order to collect the fluorescence light, detected by a PIN photodiode, are provided as well as a narrow band interference filter to separate the emission lines at 1.064 µm. The RG1000 filter is used to suppress the non absorbed pump light when measuring the integral fluorescence.
A two channel oscilloscope for displaying the time resolved signals is necessary.
The temperature and current stabilisation of the laser diode to vary the parameters of the laser diode is done by the control unit LDC-01. The unit will also be used for modulating the injection current of the laserdiode.
A comprehensive work booklet explains theoretical aspects and detailed experimental techniques.

Examples of investigation and measurement

The term scheme of the Nd ions within the YAG host-crystal is the most essential detail of this experiment. Since this material is one of the most important lasing media, the discussion of the term scheme is already a preparation for later applications. The absorption transitions shown on the left side are used experimentally. By the optical pumping of these transitions, further levels are populated. The fluorescent radiation of these levels is measured statically and as a function of time. In this regard, special attention will be given to the line of 1064 nm. Starting from the well known spectral data of the absorption transitions, the emission wavelength of the laserdiode can be calibrated. By modulation, respectively by a periodic switching in and out of the laserdiode, the decay of the fluorescent radiation in time can be observed. Since the life-time of the initial level is about 250 ms for the main line, the decay curve can easily be recorded with the help of a simple oscilloscope and the lifetime can be determined quantitatively.

Absorption spectrum
The dependence of the emission wavelength of the laserdiode on its temperature and to a certain extent also on the injection current can be used to study the spectral absorption behaviour of the Nd:YAG. By the measurement of the laserdiode radiation transmitted by the Nd:YAG rod, one gets the figure shown on the left side. To prolong the lifetime of the laserdiode, the highest temperature has been limited to 40° C in this experiment.

Spectral characteristics of the laserdiode
The maximum absorption is characterised by a significant reduction of the laserdiode radiation at the exit of the Nd:YAG crystal. Under these circumstances, the wavelength of the laserdiode is clearly determined. If the injection current and the temperature of the laserdiode are now modified such that the maximum absorption is maintained, one gets the values shown on the left side. From this, the dependence of the laser diode’s emission wavelength on temperature and injection current can be determined.

Output power of the laserdiode
The characteristic data of the laserdiode can be measured in relative units. If a laser power meter is available this can also be done in absolute units. Output power as a function of injection current, slope efficiency and threshold current are determined.

Required Equipment

Cat. No.	Qty.	Description	Illustration
02.0500	1	Profile rail 500mm The main components of the experimental systems are the optical rails OCM 650. They are manufactured distortion-free and are of thermally stabilized aluminium. The surface is electro-polished and black anodized. Because of the precise manufacturing, the smoothness deviation is less than 25 µm/m and the deviation of the symmetry axis of the rail is less than 10 µm/m, thus maintaining the optical axis during displacement of the carrier. The rail has a dovetail like profile. Gear racks can be inserted and fixed into the slots. The profile rails are available at various lengths up to 2 m.
02.2063	1	Mounting plate RMS Mounting plates are used to hold optical mounts. A characteristic feature of the mounting plates is the “click” mechanism of the inserts based on spring loaded spheres. Snapping in the groove of the inserted click mount, the optical element is kept in an exact position. On the other hand, the system allows a quick and easy change of the mounted inserts. The mounting plates are made out of special anodized aluminum. Mounted onto the carrier 20 mm, the mounting plates can be placed onto an optical rail.
02.2126	3	Mounting plate click 25 Mounting plates are used to hold optical mounts. A characteristic feature of the mounting plates is the “click” mechanism of the inserts based on spring loaded spheres. Snapping in the groove of the inserted click mount, the optical element is kept in an exact position. On the other hand, the system allows a quick and easy change of the mounted inserts. The mounting plates are made out of special anodized aluminum. Mounted onto the carrier 20 mm, the mounting plates can be placed onto an optical rail.
02.2202	1	Filter plate holder FH 650 for 3 filters 50 x 50 x 3 mm including carrier 30 mm The holder can support a total of three filter plates with the dimension of 50x50 mm. The maximum thickness can be 5 mm. Lateral springs fix the filters within the holder. Round filters can be used by means of filter adapter.
02.2526	1	Target screen in 25 mm click mount To align a light beam coaxial to the centre axis of the rail set-up this target is used as visual aid. It is mounted into a click 25 mm mount.
02.3022	1	XY-adjustment holder OCM 650 with 25 mm mount and carrier 20 mm This frequently needed component is ideal for the fine adjustment of lenses, microscope objectives, diode laser, etc. with respect to the optical axis of the rail set-up. The displacement area is 5x5 mm. Different mounts can be attached to the adjustment holder. This model provides a holder for 25 mm cylindrical components.
04.0010	1	Microscope objective x 10 with RMS thread The figure on the right shows the microscope objective screwed into an adjustment holder, however, it comes without the holder. The objective has a magnification of 10 and a numerical aperture of 0.3 and it is commonly used to collimate the divergent light emitted by laser diodes.
04.0050	2	Biconcave lens f=60 in click 25 Different glass lenses are mounted onto a special anodized aluminum click mount 25 mm by two threaded mounting rings to be used in connection with a mounting plate (02.2126).
04.0120	1	Interference filter 1064 nm with holder 50x50x5 mm Interference filters are designed as very narrow band pass filters and are commonly used to transmit only a small wavelength range. This filter has a centre wavelength of 1064 nm and a bandwidth of ±50 nm. It can be used to separate either fluorescence radiation or the laser emission from radiation which does not lie within the bandwidth of the filter. The round filter is mounted into a plate with a size of 50x50 mm and a thickness of 3 mm fitting into the filter plate holder (02.2202).
04.0122	1	RG 1000 Coloured glass filter This filter suppresses radiation with a wavelength smaller than 1000 nm in such a way, that for instance the pump radiation of 810 nm is suppressed and the generated wavelength of 1064 nm of the Nd:YAG laser transmitted. It has characteristics like a high pass filter, a size of 50x50 mm and a thickness of 3 mm fitting into the filter plate holder (02.2202).
04.0302	1	Infrared display card 0.8-1.2 µm To convert invisible radiation in a wavelength range of 0.8-1.2 µm into visible light, this card is used. Depending on the incident power, the visible spot ranges from orange to white. This card can only be used for non-focused optical power up to 0.5 W.
05.0216	1	DIMO 810 Diode laser module The diode laser module contain different laser diodes with various output power and wavelength. The wavelength is subject to certain deviations within the range as given in the table below. The integrated Peltier element allows a temperature control in the range of 15 to 40° C by means of the control unit LDC-01. Each laser diode is firmly connected to its own controller.
07.0003	1	Set of 3 BNC Connection leads BNC cable with a length of 0.8 m with attached BNC connectors on both sides
07.0102	1	PIN Si Photo detector BPX 61 complete with housing In a housing a PIN Si photo detector is mounted. Via a BNC connection the signal is fed to the respective pre-amplifier or oscilloscope. The module is clicked into the mounting plate, where it is fixed by means of three separate spring loaded balls which snap into the groove of the detector housing.
07.0200	1	LDC01 Laserdiode controller The controller unit LDC01 provides reliable and save operation of the expensive laserdiode. It contains the control circuits for power monitoring, temperature control and current setting. The values for the temperature and the diode current can be read from two large-format LED displays on the front panel of the unit. The desired values for the temperature and current can be set with precision multi-turn potentiometers. At BNC sockets on the rear of the device, analogue output signals of the temperature and current as well as the synchronising signal of the internal modulator and the photodiode amplifier of the external photo detector are provided. Via a BNC socket, an external modulator can be connected. In addition the controller contains an internal modulator for modulating the laser diode output power for investigation of the dynamic behaviour of the pumped laser as spiking, measuring of lifetimes, etc.
09.0014	1	Nd: YAG rod mounted in XY-adjustment holder This module contains the absorbing, respectively the emitting material: the Nd:YAG rod. The rod is mounted in a disk which is inserted into a click mount. As for the other optic mounts, mounting into the adjustment holder is done without use of tools. By means of an XY-adjustment the YAG-rod is aligned with respect to the optical axis. Additional fine pitch screws for angular adjustment provide alignment perpendicular to the optical axis.
10.0010	1	EXP 01 Manual	No illustration
Required Options:
19.0140	1	Dual trace oscilloscope 100 MHz Features: Frequency Range: 150 kHz ~ 100MHz Fully Digital Phase Locked Loop Technique Design High Frequency Stability: ±10ppm High Input Protection Level: +30dBm, ±25VDC Reference Level Range: -30dBm ~ +20dBm
Options:
09.0019	1	Set of spare parts	No illustration