Products

Laser Diode

Items and Definitions

1. Absolute Maximum Ratings for Laser Diode

Specifications are given for a case temperature Tc of 25 °C.

Item Symbol Definition of maximum rating Remarks
Optical
power output
Pomax

Maximum allowable output defined for continuous operation or pulse operation. This optical power output or less does not have the degradation as shown in the figure.

Graph Po/If
 
Reverse
voltage
VR Maximum allowable voltage when the reverse bias is applied to the semiconductor laser and the photodiode.  
Operating
temperature
Topr Allowable temperature when the device is operated. It is given for the case temperature  
Storage
temperature
Tstg Allowable ambient temperature when the device is stored.  

2. Laser Diode Electrical and Optical Characteristics (1)

Item Symbol Definition Remarks
Oscillation
start current
Ith

The diagram below can be divided into two regions; a spontaneous emission region A and a laser oscillation region B.The oscillation start current is called "threshold current".

Graph Ith, Iop
 
Operating
current
Iop Forward current required to maintain the specified optical power output.  
Operating
voltage
Vop Forward voltage at the specified optical power output.  
Wavelength λp

It is the peak oscillation wavelength when the laser diode is operated at the specified optical power output. In the case of plural spectra, this is the wavelength of the center line of the set of spectral lines that have 1/2 of maximum intensity. The laser oscillation spectrum is classified into two kinds, that is, single longitudinal mode or multi mode according to the feature.

発Graph Single and Multi mode
 

2. Laser Diode Electrical and Optical Characteristics (2)

Item Symbol Definition Remarks
Monitor current Imon Current value which flows when the specified reverse voltage is applied to the photodiode at the specified optical power output Po.  
Radiation
angle
Perpendicular to junction θ⊥

Light emission from the laser diode extends as shown in Fig. (a). The measurement results of the light intensity along the X and Y axis are shown in Fig. (b) and Fig. (c). The width that have 1/2 of peak intensity (FWHM) are called extension angle θ// in the horizontal direction and extension angle θ⊥ in the vertical direction. The unit is degrees.

Figure (a), (b), (c), (d)
Parallel to junction θ//
Asymmetry ΔSr

2. Laser Diode Electrical and Optical Characteristics (3)

Item Symbol Definition Remarks
Emission
point
accuracy
Positional
accuracy
ΔX
ΔY
ΔZ

Displacement of the laser diode emission point in the direction of X, Y and Z. ΔX and ΔY indicate the displacement from the center of the package, ΔZ indicates the displacement from the reference plane of the package.

Figure Emission point accuracy,Optical axis

ΔΦ⊥ indicates the optical axis deviation from perpendicular plane to the junction plane.

The emission point accuracy causes the variation of the optical coupling efficiency, which is provided by the radiation angle and the collimator, and becomes the factor of instability of other items.
Angular
accuracy
ΔΦ⊥
ΔΦ// Figure Optical axis

ΔΦ// indicates the optical axis deviation from parallel plane to the junction plane.

 
Differential
efficiency
ηD This indicates the value of the incremental change in optical power output for an incremental change in forward current; represented by the slope in the region B in the figure of "Optical Power Output vs. Forward Current Characteristics". The larger the differential efficiency, the steeper the curve of the optical power output vs. current characteristics becomes. The smaller the differential efficiency, the gentler the curve becomes. Because the larger the differential efficiency, the steeper the curve of the optical power output vs. current characteristics, the width between the oscillation start current and the rated operating current becomes small. On the contrary, because the smaller the differential efficiency, the gentler the curve of the optical power output vs. current characteristics, the width between the oscillation start current and the rated operating current becomes large.

2. Laser Diode Electrical and Optical Characteristics (4)

Item Symbol Definition Remarks
Astigmatism As Apparent emission point deviation on the planes between parallel and perpendicular to the P-N junction plane in the direction of the optical axis.  
Polarization
ratio
P1 Ratio of the optical power of the linearly polarized component parallel to the P-N junction plane to that of the linearly polarized component perpendicular to it.  
Relative
intensity
noise
RIN Ratio of the optical intensity noise to the average optical power output per unit frequency. Actually, the light fluctuations are measured and calculated using a photodetector such as a photo diode and an amplifier. Therefore, RIN is obtained by eliminating the shot noise generated by the photodetector and the thermal noise generated by the amplifier from noise detected in the specified frequency, and dividing the remaining noise by the average output of the semiconductor laser (generally detected by the optical current of the photodetector) and the measurement frequency bandwidth.  
Photodiode
dark
current
ID Current value flowing to the optical monitor output when the semiconductor laser is not in the emitting state.  

Handling Precautions and Reliability

Laser Diode Handling Precautions

For laser diodes, unlike ordinary transistors and integrated circuits, it is necessary to pay special attention to the following points when handled.

  1. Eye protection against laser beams
  2. Gallium arsenide
  3. Electrical stress
  4. Contamination (dust and dirt)
  5. Mechanical stress
  6. Thermal stress
  7. Condensation

All these precautions are collectively described below. :

https://www.sony-semicon.co.jp/e/csr/quality/#handbook

"Chapter 5 Notes for Each Product Category and Other Noteworthy Items", "5.2 Laser Diode"
When handling a laser diode, read through the precautions and use the laser diode safely in the correct way.

Measurement Method

1. Optical Power Output vs. Forward Current Characteristics (L-I)

The optical power output vs. forward current characteristics are measured by supplying current in the forward direction of the laser diode and detecting the generated laser light with a photodetector whose size is large enough.
Laser diodes are classified into DC (CW: continuous wave) drive and pulse drive, depending on their drive circuits.
Photodetectors in general use are silicon PIN diodes or APDs (Avalanche PhotoDiode) calibrated with a reference photodiode.

Figure 1

2. Longitudinal Mode Spectrum

A longitudinal mode spectrum shows the results of laser light separated into components by an optical spectrum analyzer, etc., and expressed in terms of the wavelength vs. light intensity distribution.
Spectrum where many laser emission modes are excited are called multi-longitudinal mode (multi mode) emission, whereas spectrum where only a single laser emission mode is excited are called single-longitudinal mode (single mode) emission.
The wavelength of a laser diode shifts toward the longer wavelength side as the ambient temperature rises. Therefore, for high accuracy spectrum measurement, the temperature of the laser diode must be controlled by a Peltier element, etc., to maintain its constant level.

Figure 2

3. FFP (Far Field Pattern)

A far field pattern is measured by detecting the vertical and horizontal light intensity distributions via a slit to junction of a laser diode.
The spatial resolution of a far field pattern image depends on the distance between the laser diode and the photodetector, and also the slit width.

Figure 3

4. Astigmatism

The difference between the virtual light source positions of beams in parallel and perpendicular directions to the junction is the astigmatism. It is the one of the simplest method for detecting the virtual light source position to measure the beam diameter at the point closest to the laser end and to find the position where the beam diameter is minimized while moving the laser diode. The astigmatism of a laser diode produces large effects on its imaging performance. Therefore, it is particularly an important item for readout operation of optical discs with high recording density.

Figure 4

5. Polarization Ratio

The polarization ratio of a laser can be calculated by converting the light emitted from the laser diode into parallel rays with a collimator lens and by finding the ratio of the maximum to minimum value of the quantity of light while rotating a polarized beam splitter. The polarization ratio generally has different values, depending on the aperture diameter of the collimator lens and the optical power output of the laser diode. Therefore, it is necessary that the measurement conditions that match the operating conditions are set.

Figure 5
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