Good design drives diode performance

12 Jan 2004

If you plan to buy a laser diode driver, make sure you check the specifications with care. Martin Lawrence believes that manufacturers could learn a lot from the hi-fi community.

From Opto & Laser Europe January 2004

Anyone who has just spent several thousand pounds on a semiconductor laser diode should think twice before simply connecting it to a conventional laboratory power supply. To avoid damaging your laser diode and ensure that you obtain an optical signal that is stable with minimum noise, it is essential that you connect it to a good-quality power supply that is designed for the job. This specialized piece of electronics is commonly known as a laser diode driver.

A laser diode driver is essentially an ultra-low-noise current source that is designed to drive a very low impedance load (the diode). In fact, it has a great deal in common with a good-quality hi-fi music preamplifier driving 4-8 Ω speakers. And, like hi-fi amplifiers, diode drivers usually come with a host of special operational modes and safety features.

There is a wide range of drivers on the market, designed to supply everything from just a few milliamps for a telecoms laser to several tens of amps for a high-power diode stack. All of them, of course, would probably claim to be the best product available. However, a small amount of investigation can reveal big differences between apparently similar instruments.

The key features of a good current source are good design and careful attention to build quality. This is something that manufacturers of hi-fi amplifiers seem to have embraced but that has been sadly neglected by some makers of laser electronics.

Making your selection The first problem facing you as a potential buyer is that makers will seldom provide you with a complete circuit diagram of the driver, which would enable you to make a proper assessment of its design. (It is worth asking, though.) To make matters worse, unlike hi-fi amplifiers, there are no set specifications and sometimes meaningless performance figures are given, especially for noise, which is unfortunately one of the worst-reported figures in sales literature. Noise should be presented as dB/root Hz.

My advice to anyone buying a laser diode driver is to study the specifications carefully and only buy from a manufacturer who provides a comprehensive list of well-defined terms. At the very least, the maker should be able to specify the power delivered, the noise in dB/root Hz and a measure of the bandwidth and stability of the direct current supplied.

If the driver offers the facility to pulse the laser by modulating the drive current via an external control signal, additional specifications are required - namely, the input impedance of the modulation channel and acceptable limits for the amplitude and frequency of the external signal. Any laser diode driver on the market will generate sufficient potential difference (voltage) to drive the laser into conduction (light emission), so the key performance issues are how stable the set current is and how clean the signal sent to the laser is. Central to this performance is the signal-to-noise ratio (SNR) of the driver's output current.

For SNR values to be credible there must be a properly defined figure for the noise generated by the driver. Noise should be universally defined as the logarithmic ratio of the amount of unwanted signal to wanted signal within a defined bandwidth.

A current source might have the potential to offer an SNR of 80 or 100 dB, but once burdened with an inadequate central power supply and noisy digital controls that are poorly earthed (see below) the real SNR may drop to 50 dB or less. Although this may be acceptable in some cases, it can lead to a broadening of the laser's emission wavelength and a reduction of its coherence length, especially in the case of external-cavity lasers. Sometimes when this occurs, users mistakenly think that the diode is at fault rather than the power supply driving it.

Design points: proper earthing A well-thought-out earthing scheme is essential for good driver performance. Starting at the front panel, if the current source controls are digital, as most are today, the digital parts of the circuit should be referenced to a separate earth line on the printed circuit board (PCB). The same goes for the low-power signal circuitry, the power amplifier and the read-out meters.

Years ago, hi-fi makers discovered the benefits of "star" earthing techniques, in which different functions have separate earth lines that are tied together at a single point on the power supply. However, many laser diode driver suppliers still tend to design a current source, specify it, and then afterwards add additional control and read-out circuitry with the earthing points randomly distributed on the PCB.

Needless to say, such a disorganized approach does not result in the same noise performance as the original core, although its specification is the one that is usually printed. The worst crime is connecting a read-out meter to the laser return path. As the meter measures the output drive current it dumps a nasty current spike directly into the laser drive.

Following on from the earthing scheme, another factor that influences performance is the quality of the power supply within the driver. The power supply should provide more than sufficient current and should ideally have more than one output. This allows the laser drive circuitry to be operated from a separate power rail to the digital control-and-measurement circuitry.

The benefit of this is better isolation of potentially noisy electronics from the low-noise current supply to the laser diode. Again, as was discussed with earthing techniques, the only way you can discover the quality of the power supply ahead of making a purchase is by examining the circuit diagram.

Testing To test whether a driver is well-designed, try putting a 2.2 ω, 7 W resistor across its output. Change the current up and down and use an oscilloscope to observe what happens to the voltage across the resistor. The trace response should be smooth. If it is not and you see spikes or a lot of noise, you have a badly designed driver on your hands.

In theory, an audio comparison could provide an interesting test of current sources. Replacing the internal voltage reference diode in a laser-diode driver with an audio input (say from a compact disc player) and connecting the output to a pair of headphones would enable the purchaser to listen to the performance of the current source.

If the sound quality is poor and the presence of any kind of hiss or crackle can be detected, the design is lacking. Admittedly this is not a quantitative test, but the human ear is a very discriminatory device. The bandwidth, clarity of signal and resistance to drift are just as important in driving laser diodes as in driving audio speakers. So is the elimination of spiky background noise.

Room for improvement Laser diode drivers are a relatively small but still important section of the electronics market. However, the variation in quality of the products available is still unacceptably large and in terms of value for money there is definitely room for improvement. Some very good units are available for large-drive-current (more than 1 A) and original-equipment-manufacturer applications, but in the off-the-shelf, 0-1 A arena nothing currently available comes close to hi-fi preamplifiers in either quality or price.

Today, hi-fi amps will almost certainly offer at least 100 dB SNR, as well as constant (and levelled) output power for a range of impedance loads. They also feature high immunity to mains-borne noise and £100 or so buys a very competent unit with two independent output channels. Laser diode driver makers would do well to follow their example.

Top tips when buying 1. Ask for a copy of the driver's circuit diagram.

2. Check the specifications carefully: are they comprehensive and properly defined?

3. Try before you buy: ask for a demonstration of the resistor test.

4. If you want to pulse the laser diode, check out the driver's modulation options.

Laser diode driver features explained Below is an explanation of some of the common features found in a laser diode driver.

Constant current mode The usual mode of operation, in which the driver stabilizes the current supplied to the laser diode.

Constant power mode In this mode the driver uses a feedback loop to stabilize the laser diode's output power. A signal from the laser diode's internal monitor photodiode (if equipped) is monitored and used to control the value of the drive current. Care should be taken when using this mode. If a proper connection to the monitor photodiode is not made then the driver may try to drive a large current through the laser diode.

Modulation options Many drivers offer an input port for an external modulation signal. This feature allows the current supplied to the diode to be modulated so that it emits pulses of laser light. Some more expensive models have internal circuitry for modulating the current supplied to the laser.

Safety features Well-designed drivers incorporate several safety features to protect laser diodes from damage. These may include a current limit, which enables the user to enter a maximum limit for the drive current which cannot be exceeded; transient protection circuitry, which stops potentially deadly current or voltage spikes from being transmitted to the laser diode; and start-up/shut-down procedures that mean that the current delivered to the laser diode is smoothly ramped up or down when the driver is turned on or off.

Temperature control Often it is desirable to stabilize or tune the temperature of the laser diode to optimize its performance. Although separate instruments - known as thermoelectric cooler controllers - are available to fulfil this need, some more expensive laser diode drivers combine both a current source and a temperature controller into a single product.