Available Photonics Experiments:

P5875 Optical time domain reflectometry OTDR
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  • Short pulse laser diode
  • Optical fibre
  • Coupling light to fibre
  • Si PIN fast photodetector
  • Speed of Light
  • Light echoes
  • Fault detection
  • Attenuation of optical fibre
  • Examples of investigations and measurements
Principle of operation
The emission of a pulse laser diode (A) with a peak power of 70 W and a pulse width of 100 ns is collimated (B), passes the polarising beam splitter cube (PBSC) and is launched into the fibre (F) by the coupling optics (C). Micro structures which are more or less distributed homogeneously inside in every fibre and are a result of the manufacturing process. Radiation which impinges on these structures disperses in such a way that the scattered light also reaches back to the entrance of the fibre. The back scattered light is deviated by the PBSC and focused to the photodetector (P). The quarter wave plate (Q) modifies the polarisation to obtain maximum signal at the detector.
OX General set-up
The pulse diode laser (4) is mounted into a 4 axes adjustment holder. The objective (13) collimates the strong divergent emission of the laser diode. The almost parallel light passes the polarising beam splitter cube (18) and is launched into the fibre by the objective (15). The fibre is placed into the groove of the holder (14) and fixed by magnets. The returning light has a different polarisation state and passes the rotary quarter wave plate (6) to achieve the highest degree of reflection at the beam splitter. The returning light is focused onto the fast photodetector (8). The signal is available at the BNC output of the signal conditioner box. The diode laser controller (7) allows the setting of the energy per pulse as well as the repetition rate up to 2.5 kHz.
OX Characterising the pulse diode laser
In a first task the properties of the pulsed diode laser are studied. For this purpose only the modules are shown in the figure left are required. The peak pulse is measured as function of the load voltage and repetition rate. By using the data of the laser diode the peak power can be determined.
The diode laser (A) can be rotated within the holder (H) and the polarisation recorded for different angle positions. For the OTDR measurements it is important that the diode laser is orientated in such a way that the maximum of intensity passes the polarising beam splitter (B).
OX OTDR Measurements
With the completed setup as already shown in the general setup the OTDR measurements performed using the optical multimode fibre having a length of 1000 m. Thus approximate time of flight is 2 x 5 µsec which can be monitored with a 100 MHz oscilloscope.
The figure on the left shows the exponential decay of the back scattered light. This is related to the losses along the fibre. Therefore exponential part of the curve contains the information about these losses. From the slope of the logarithmic intensity curve the absorption can be calculated.
  • P5875 Optical Time Domain Reflectometry (OTDR) consisting of:
12E-02401optical fiber cleaver and breaker S315
22E-02501Adjustable plastic cover stripper 103-S
33L-02002BNC cable, BNC connector both sides, 1,5 m
44B-01701 Optical glass fibre, length 1000 m, core 50 µm multi mode
54B-06201Biconvex lens, f=60 mm, free opening 18 mm, click 25
64B-08701Quarter wave plate, click 25
78X-XP211Manual OTDR
8ED-00901Pulsed laser diode controller PLDC-01
9ED-03101Photodetector, ultrafast with amplifier
10LS-13001Pulsed diode laser module
11MC-00031Profile rail MG-65, 300 mm
12MC-00081Profile rail MG-65, 800 mm
13PM-00701 Infrared display card, spectral range 0.8 -1.6 µm
14XM-00361Module B - Collimating optics on carrier
15XM-05002Bare fibre holder with carrier 30 mm
16XM-05101Coupling optics, microscope objective x 20 with XY- adjustment holder
17XM-10903Mounting plate C25 with carrier 20 mm
18XM-11101Adjustment holder, 4 axes, carrier 20 mm
19XM-21001Beam splitter module
 Required Options:  
 TP-01001Oscilloscope 100 MHz digital, two channel