Laser Diode Characteristics, Precautions for Use and Drive Circuit Designs

Laser diodes (LD) are semiconductor devices that convert electrical energy into high-power optical energy. These devices are currently used in the fields of telecommunications and medicine and in industrial cutting and welding applications. This article discusses the characteristics common to laser diodes, such as high coherence, narrow spectral width and high directivity, while also explaining and defining these terms. Precautions required to avoid excessive currents, static electricity and heat generation are detailed and the drive circuits associated with such diodes are described.

Laser Diode Characteristics

This section explains the basic characteristics of laser diodes along with the terms and symbols used in datasheets to indicate these characteristics. The package internal configurations and circuitry are additionally examined.

Basic Characteristics of Laser Diodes

1. Optical Power

The optical power value, Po, is the most basic characteristic of a laser diode. This parameter is defined as the light output intensity in the case that a specific current is applied to the device in the forward direction, and is typically expressed in units of W. This is shown on a graph as the I-L curve (optical power (L) – forward current (IF) characteristics). As can be seen from the I-L curves, increases in temperature reduce the optical power that can be obtained at a given current. That is, greater current values are required to obtain a specific optical power as the temperature is raised. As an example, the data shown below indicate that the current required to obtain an optical power of 5 mW is 30 mA at 25 °C but 44 mA at 70 °C.

Example: RLD65NZX2

At present, laser diodes with optical power ranging from several milliwatts to several hundred watts are commercially available. It is important to select a laser diode with the appropriate optical power based on the intended application.

2. Lasing Wavelength (Oscillation Spectrum)

The lasing wavelength λ (expressed in units of nanometers) is an indicator of the frequency (or color) of the light emitted by a laser diode and is another important characteristic of such devices. Shorter wavelengths are closer to ultraviolet (UV) and blue light, while longer wavelengths are closer to infrared (IR) light. Different wavelengths have different propagation characteristics when employed in communication applications, in terms of the extent to which these wavelengths are absorbed or reflected by certain substances. Hence, a laser diode producing an appropriate wavelength for a given application must be selected. The effects of temperature and optical power on the lasing wavelength should also be considered. Specifically, as the temperature of the junction (that is, the active layer) of the laser diode chip increases, the resonator length physically extends and the refractive index increases. In addition, the case temperature tends to rise and the lasing wavelength increases as the optical power increases.

3. Far Field Pattern

Although laser light is often thought of as a straight, parallel beam, the light emitted from a laser diode actually diverges to some extent as it diffracts. The light beam at some distance from the end surface of the laser diode will therefore exhibit an intensity distribution known as a far field pattern (FFP). As the resonator (active layer and stripe) in the chip is built up to several tens of nm in the vertical direction and several µm in the horizontal direction, the far field pattern is generally larger in the vertical direction relative to the active layer than in the horizontal direction.

Definitions of Terms and Symbols for Laser Diode Characteristics

●Absolute Maximum Ratings:
The values that must never be exceeded, even momentarily, under any external conditions. These values are specified when case temperature Tc = 25°C.

●Optical power (Po (max.)):
The maximum permissible output power during continuous operation. No kinks will appear in the optical power vs forward current characteristics up to this value. (See Fig. 1)

Figure 1

●Reverse voltage (Vr):
The maximum allowable voltage when a reverse bias is applied to the device. Lasers and photodiodes (PD) are rated separately.

●Operating temperature (Topr):
The ambient temperature allowed when the device is in operation. Defined by the case temperature of the device.

●Storage temperature (Tstg):
The ambient temperature allowed when the device is stored.

●Characteristics (Electrical and Optical Characteristics):
It should be noted that the laser diode properties provided in the Characteristics table in a datasheet are those associated with the conditions described in the Conditions column.

●Threshold current (Ith):
Figure 2 provides a typical plot from which the threshold current can be obtained. Here, A is the spontaneous emission region and B is the laser oscillation region. The threshold current is the current at which the laser oscillation starts and is defined by the intersection of the X-axis and the extrapolation of the optical power curve in the laser oscillation region.

●Operating current (Iop):
The forward current required to generate a specific optical power.

●Operating voltage (Vop):
The forward voltage at which a specific optical power is generated.

●Differential efficiency (η):
The average increase in the optical power per unit drive current. The slope of the I-L curve in the laser oscillation region (see Fig. 2).

Figure 2

●Monitor current (Im) :
The output current of a photodiode generating a specific optical power when a specific reverse voltage is applied to the photodiode for monitoring purposes.

●Parallel beam divergence (θ//) and perpendicular beam divergence (θ⊥):
As discussed above and shown in Fig. 3, a laser beam will diverge as it moves through space. Figure 4 shows the resulting intensity distribution in the horizontal (X) and vertical (Y) directions with respect to the junction surface. The widths of each distribution at one half of the peak height (otherwise known as the full width at half maximum) are referred to as the parallel and perpendicular beam divergences and are expressed as angles.

Figure 3: Radiation Characteristics

Figure 4: Radiation Characteristics

●Parallel beam tolerance (Δϕ//) and perpendicular beam tolerance (Δϕ⊥):
These values refer to the deviations of the optical axis with respect to the reference plane and are defined as (a-b)/2, as shown in Fig. 5, for both directions (see Fig. 4).

Figure 5: Deviation Angle

●Emission point accuracy (ΔX, ΔY, ΔZ):
The extent of misalignment of the emission point from the center of the package (ΔX and ΔY) and from the reference plane (ΔZ) (see Fig. 6).

Figure 6: Emission Point Accuracy

●Peak emission wavelength (λp):
This parameter is the wavelength at which the emission spectrum exhibits the maximum optical intensity. The emission spectrum used to obtain this value can be either single mode or multimode, as shown in Fig. 7. In the case of a multimode spectrum, the peak emission wavelength is defined as the wavelength of the highest peak.

Figure 7: Emission Spectrum

●Power conversion efficiency (PCE) :
The ratio of the output optical power to the input electrical power.

●Astigmatic difference (As):
The focal points of the light emitted by a laser diode relative to the junction surface will differ in the horizontal and vertical directions, and the distance between these two focal points is defined as the astigmatic difference (As). This value should be kept to a minimum, as a large As can significantly affect the quality and shape of the laser beam.

Laser Diode Package Internal Configurations and Circuitry

Laser diode packages are available with or without integrated photodiodes used to monitor the laser diode as a means of maintaining a constant optical output. ROHM refers to the pins of a three-pin package as pins 1, 2 and 3, clockwise when viewed from the top of the package (the side where the laser beam is emitted). Pin 3 is typically referred to as the common pin as this pin is shared by the laser diode and photodiode.

[An example of a 3-pin package.]

The polarity of the laser diode and of the photodiode (comprising the internal circuitry of the package) may vary between products. As an example, ROHM’s laser diodes are named using alphanumeric characters according to the scheme shown below.

Precautions for Use of Laser Diodes

Laser diodes are very sensitive devices and several precautions must be taken when using these diodes. Among these precautions, the most important include remaining below the absolute maximum rating, employing surge and electrostatic discharge (ESD) protection and using an appropriate cooling system to ensure temperature control. These devices can also be dangerous and so it is important to follow safety protocols.

Absolute Maximum Ratings

When operated beyond their maximum ratings, laser diodes can be instantly destroyed or degraded, significantly reducing product reliability. Therefore, it is vital not to exceed the specified maximum ratings even momentarily. In particular, the following points should be noted.

（1）Elements may be destroyed by current surges generated when power to the device is switched on and off. The transient characteristics of the power supply should be assessed and it should be confirmed that the current surge will not be above the maximum rating.

（2）The maximum rating is typically that for a case temperature of 25 °C. At higher temperatures, the maximum optical power and allowable power dissipation both decrease, limiting the operating range of the diode.

Prevention of Static Electricity and Surges

Laser diodes are prone to catastrophic optical damage (COD) when subjected to current surges such as may be produced by static electrical discharge. In fact, the ESD tolerance of these devices is much lower than that of other common discrete electronics. In the case that a current surge enters the diode in the forward direction, excessive emission will occur that may damage the light-emitting area and end face, resulting in reduced emission efficiency and, in a worst-case scenario, complete loss of operability. Because COD is a serious and permanent phenomenon that can occur in a short duration, preventive measures are important.

＊ESD：Electro Static Discharge
＊COD：Catastrophic Optical Damage

Key Factors Preventing COD

Electrostatic damage to a laser diode is often a result of a current surge resulting from a static electrical discharge generated by a human body or a spike voltage associated with switching the power supply on or off. The following measures should be taken when handling laser diodes to prevent these phenomena.

1. 1.Working Environment
   Ground the equipment and associated circuitry while restricting noise from the grounding line and use noise filters, noise-cut transformers or other preventive devices at each power input section.
2. 2.Operators
   Anyone handling such devices should wear anti-static clothing, caps and shoes along with a grounding strap having a 1 MΩ resistance.
3. 3.Transport and Storage Case
   Use antistatic materials.
4. 4.Other
   Avoid using a laser diode near fluorescent glow lamps because the laser may be degraded or destroyed by induced surges when in proximity to equipment that generates radio frequencies.

Heat Protection (Heat Dissipation)

As is the case for all semiconductor devices, a laser diode generates heat at junctions during the prolonged application of a current, such that the diode temperature increases. Without sufficient heat dissipation, the case temperature will rise and the optical power will decrease, requiring more current to be applied to maintain the specified optical power. An increase in the forward current further increases the case temperature, which in turn increases the forward current. To prevent this scenario, a heat sink having dimensions of 30 × 30 × 3 mm or larger and made of aluminum or a similar material should be employed, in close contact with the stem of the diode.

Safety

The light emitted from a laser diode can be very dangerous if used incorrectly. In particular, looking directly at the emitted light or viewing the light through a lens can cause retinal damage. Hence, an infrared camera (ITV) or similar device should be used for optical axis alignment. In addition, a high power laser beam can cause irritation or burns when contacting the exterior of the body and so should never be directed at a person.

ROHM laser diodes are classified as Class IIIb and IV according to optical power and wavelength. The laser diode datasheets and the bags in which the products are packed have warning labels shown below.

Package Handling

Diodes should not be dropped or subjected to excessive pressure. Care should be taken not to damage the glass seal or to break the wires when bending the leads.

・Packages with a glass window
The glass window of the laser diode should never be touched. Any scratches or stains on the glass window will alter the optical characteristics of the laser.

・Open packages
An unfavorable environment may degrade the characteristics and reliability of a diode. The device should be protected from contact with foreign materials such as toner, smoke, corrosive substances, volatile components of glues or fluxes, and condensation. The effect of optical tweezers should also be considered. In addition, it is important to avoid touching the components inside the device cap, including the laser chip emission component.

Polarization Characteristics

Polarized light is characterized by the oscillation of electric and magnetic fields in a single plane and the light emitted by laser diodes is polarized. In the case that the electric field of the light beam oscillates in a plane parallel to or perpendicular to the junction surface of the laser diode chip, the device is said to operate in the transverse electric wave (TE) or transverse magnetic wave (TM) mode, respectively. Because different devices may operate in one mode or the other, care should be taken when using polarization optics.

Optical Power Measurement

The optical power of a laser diode can be ascertained by quantitatively measuring the intensity of the optical signal using a meter. The procedure is as follows.

Measurement Preparation

a. The optical power meter is set up and calibrated for the wavelength to be used.
b. The laser diode is operated in a stable temperature environment with a heat sink attached and using a temperature controller.

Measurement Procedure

a. The light-receiving surface of the optical power meter is adjusted such that the entire light flux of the laser beam irradiates this surface.
b. The light-receiving surface of the meter is tilted by 5° to 20° with respect to the optical axis to prevent the light reflected from this surface from returning to the laser diode.
c. An electric current is applied to the laser diode and the output optical power is measured.
d. The optical power is recorded and, if necessary, an I-L plot is generated.

Relationship between Injection Current and Light Output
The injection current (I) – light output (L) characteristics of a device reflect the relationship between the forward current (IF) and the optical power output (PO). Based on the I-L properties of a device, the operating current (Iop) and the threshold current (Ith) at which the laser diode oscillation is initiated can be determined. The monitor current (Im) is the current at which the laser beam emitted from the rear surface of the laser chip is detected by a built-in photodiode. Assessing the I-L characteristics of a laser diode allows the performance and operating conditions for the device to be evaluated and the optimal operating conditions to be determined.

Basic Laser Diode Drive Circuits

There are two major techniques used to drive laser diodes: continuous wave (CW) and pulse drive. The pulse drive method produces a pulsed output in response to a brief current application, resulting in a very high peak output intensity.

CW Drive

A CW laser continuously oscillates to produce a characteristic stable optical power output. These devices are available in a wide range of wavelength bands, including visible and infrared light.

Major wavelength range: Visible to near-infrared light
Typical optical power: several mW to several W
Drive method : CW drive (APC, ACC, etc.)
Major applications: Laser pointers, markers, laser beam printers, optical disks, light sources for sensors

Pulsed Drives

A pulsed laser oscillates to produce output pulses of short duration. These devices may operate at a constant repetition rate or produce short individual pulses. Because high peak optical power can be obtained with these devices and laser beams can be delivered over long distances, the demand for pulsed lasers as light sources in long-range sensors has increased.

Major wavelength range: Near infra-red light
Typical optical power (peak): Several W to several hundred W
Drive method : Pulsed drive (current resonant type, square wave type, etc.)
Major applications : Light source for LiDAR especially ToF

CW Laser Diode Drive Circuits

The two major types of circuits used to drive CW laser diodes are automatic power control (APC) and automatic current control (ACC) circuits.

Automatic Power Control

This technique controls the LD drive current so as to maintain a constant optical power, based on monitoring the current associated with a photodiode built into the laser diode package.

[An example of an APC drive circuit.]

Automatic Current Control

This method applies a constant current to the laser diode.

Precautions related to ACC drive circuits:

The optical power output of a laser diode at a given current will vary with changes in temperature. An ACC circuit requires the temperature of the diode to be held constant so as to maintain a constant optical power output. Hence, an APC circuit is typically employed to ensure that the power output remains unchanged as the ambient temperature fluctuates.

[An example of an ACC drive circuit.]

Pulsed Laser Drive Circuits

A current resonant drive circuit, a type of pulsed laser diode driver device, is shown below. This type of diode is capable of delivering short pulses of light at high output power.

[An example of a current resonant drive circuit.]