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Photonics Experiments

Optics Experiments

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Diodelaser

Topics:
Binding of Molecules
Fermi Distribution
Inversion in Semiconductors
Types of Laser Diodes
Spatial Distribution
Spectral Properties
Beam Shaping
Polarisation Properties

Laser diodes differ from most “classical” lasers in two distinct ways:
Firstly, they do not posses an inherently defined wavelength. Instead of two defined energy levels, the lasing transition occurs between two energy bands.
Secondly, the pn junction defines the lasing volume, instead of the resonator in a classical laser.
The experiments investigate the variation of the emitted laser wavelength versus temperature and current. The divergence and polarisation of the laser emission are examined. The set-up comprises a 30 mW laser diode, with an integrated Peltier cooler, a mount and driver. Collimating optics, lenses and a polarisation analyser are provided, along with a photodiode detector. A Nd:YAG rod is supplied, whose absorption provides defined wavelengths (a monochromator is available as an option). All optical mounts and positioners are included. The laser diode is mounted in a rotational stage which allows the independent rotation around the beam propagation axis as well as perpendicular to this axis to measure the spatial distribution of the emitted laser light.
The polarisation for different values of the injection current is analysed by means of a polariser.
The wavelength will be calibrated by employing the well known absorption transitions of the Nd:YAG. In addition, an optional monochromator can be used to measure the change of wavelength by varying the temperature. The shift is approx. 0.25 nm per °C. The temperature range of the diode laser controller can be varied from 10 to 40° C which results in a shift of 7.5 nm.
The temperature and injection current are stabilised and displayed by the controller, which also contains an amplifier for the photo detector.
The output of the amplifier is connected to a simple digital voltmeter.
The use of an oscilloscope is recommended to suppress disturbing other light. In this case some of the measurements are carried out with modulated diode

Examples of investigation and measurement

Basics
The creation of a population inversion within a semiconductor is the main part of the chapter „basics“. The comparison to „classical lasers“ is made and the consequences with regard to the functioning of a diodelaser are pointed out. These are, for instance, the dependence of the emission wavelength on temperature and injection current as well as the spatial and spectral properties of the diodelaser radiation.

Absorption spectrum
In this experiment, the exact wavelength of the diodelaser is determined by the well known absorption lines of the Nd-ions within the YAG host-crystal. 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. 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 in a way 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. By means of the polarisation analyser the degree of polarisation of the laserdiode radiation is measured. Since the emission is a mixture of LED and diodelaser radiation, the polarisation of the laser radiation becomes dominant for higher injection currents.

Required Equipment

Cat. No.	Qty.	Description	Illustration
02.0500	1	Profile rail OCM 650, 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.
02.1402	1	Carrier platform OCM650 with horizontal and vertical rotator Inside a carrier platform with a length of 100 mm a rotator for 360° vertical rotation is integrated. On top of this a second 360° rotator is mounted so that also the rotation around the horizontal axis can be performed.
02.2063	1	Mounting plate OCM 650 RMS threading (microscope) with carrier 20 mm 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 OCM 650 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.
02.2526	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.
02.2527	1	Click mount 25 mm with 1 mm hole aperture This click mount 25 mm contains a disk with a 1 mm hole aperture and is commonly used to reduce the acceptance angle of a photo detector.
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.
04.0080	1	Cylindrical lens f=25.4 mm 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).
04.0082	1	Cylindrical lens f=80 mm 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).
04.0302	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.
04.0306	1	Optic cleaning set Especially for optics used in connection with laser applications, cleaning the optic surfaces is a must for satisfying operation of the laser. For this purpose soft cleaning tissues wetted with pure aceton are used. To hold the folded tissues clamp pliers are provided. To store the leaning liquid, a bottle with dispenser top is provided. However, due to drug administration laws this bottle comes empty and the required aceton must be provided locally.
05.0216	1	DIMO 810 Diode laser module 810 nm 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.
07.0010	1	BNC - Banana adapter connection leads 2x4 mm plugs Connection lead with one side BNC and the other side two 4 mm banana plugs
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. 09.0022 Imaging optics for light transfer from halogen lamp to wavelength selector The light of the halogen lamp is focused into the fibre bundle having a cross section transducer from a circular input to a slit output of 10 x 0.6 Mm. In front of the bundle exit, a cylindrical lens with a focal length of 10 mm collimates the emerging light.
09.0044	1	Polarisation analyser with mounting plate and rotation assembly In a rotational ring mount with a 360°, scale a polariser is assembled. By turning the ring, the transmitted intensity is monitored. In case the incident light is polarized, a minimum intensity occurs indicating that the polariser is adjusted 90° with respect to the polarisation direction of the incident light.
10.0040	1	EXP 04 manual	No illustration
19.0396	1	Digital multimeter 3 1/2 digits
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
09.0049	1	Set of spare parts	No illustration
19.0190	1	Monochromator 600 lines/mm, 400-1600 nm A quarter meter Cerny Turner monochromator is provided with a grating of 600 lines per mm. The spectral range covers 400 to 1600 nm. The robust design makes this unit useful for daily use in educational situations. The grating is driven by linearized mechanics. The wavelength is read using a mechanical display.