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IEC 62007-2: Measuring Methods for Semiconductor Optoelectronic Devices in Fibre Optic Systems
Standardized Measurement Techniques for LEDs, Laser Diodes, and Photodetectors
IEC 62007-2:2009 defines standardised measuring methods for semiconductor optoelectronic devices used in fibre optic digital communication systems. This standard covers the characterisation of photoemitters (LEDs and laser diodes) and receivers (PIN photodiodes, avalanche photodiodes, and PIN-TIA modules), providing engineers with reliable and repeatable measurement procedures essential for component specification, qualification, and system design. The standard was developed by IEC subcommittee 86C and represents the second edition of this important reference for optoelectronic device characterisation.
Semiconductor optoelectronic devices are the fundamental building blocks of optical communication networks. Standardised measurement methods ensure that device specifications are comparable across manufacturers and that system designs can be reliably based on published datasheet values.
Measuring Photoemitters: LEDs and Laser Diodes
The standard covers comprehensive measurements for LEDs and laser diodes, each designed to characterise specific aspects of device performance critical for system design. Radiant power and forward current are fundamental measurements using an integrating sphere with a calibrated detector to capture total emitted radiation regardless of emission pattern. The integrating sphere ensures that all emitted light is collected through multiple reflections from the sphere walls, leading to uniform irradiance of the detector surface proportional to the total emitted flux. For devices with optical fibre pigtails, the measurement setup must account for fibre coupling efficiency.
| Parameter | Device Type | Measurement Technique | Typical Range |
|---|---|---|---|
| Radiant power | LED/LD | Integrating sphere + calibrated detector | 0.1 mW – 500 mW |
| Small-signal cut-off frequency (fc) | LED/LD | Frequency sweep with constant AC modulation | 10 MHz – 10 GHz |
| Threshold current (ITH) | LD | Second derivative of power vs current curve | 1 mA – 100 mA |
| Relative intensity noise (RIN) | LED/LD | Noise power spectral density measurement | -140 to -160 dB/Hz (LD) |
| S11 parameter | LD/LED | Network analyser reflection coefficient | -10 to -30 dB |
Small-signal cut-off frequency measurement determines the modulation bandwidth of emitters. The forward current is modulated at a low reference frequency, then the frequency is increased while maintaining constant modulation depth until the AC radiant power drops by 3 dB. This frequency is defined as fc. For LEDs, the modulation level must be kept constant; for laser diodes, the DC bias must be maintained above threshold. Threshold current for laser diodes is determined using the second derivative method, providing a consistent, unambiguous definition that is insensitive to measurement noise when properly implemented.
When measuring laser diodes, optical feedback must be minimised to prevent measurement distortion. Reflected power re-entering the laser cavity can cause mode hopping, intensity noise, and frequency shifts that significantly affect measurement accuracy. Optical isolators with at least 30 dB isolation are recommended for high-accuracy measurements.
Receiver Characterization: PIN Photodiodes and APDs
The standard provides detailed measurement procedures for photodetectors used in optical communication receivers. Noise measurement requires careful separation of photodetector noise from amplifier and system noise using a substitution method: first measuring total noise power with the device biased and illuminated, then replacing it with a broad-spectrum source adjusted to produce the same photocurrent. The APD excess noise factor is particularly important, as the statistical nature of avalanche multiplication introduces additional noise beyond expected shot noise. This factor is directly related to the effective ionization coefficient ratio (k) of the semiconductor material.
| Parameter | Device | Technique | Engineering Importance |
|---|---|---|---|
| Excess noise factor (F) | APD | Noise power vs multiplication factor | Determines SNR degradation from avalanche gain |
| Multiplication factor (M) | APD | Photocurrent ratio at high/low bias | Defines internal current gain |
| Responsivity (R) | PIN-TIA | Output voltage vs incident optical power | Overall conversion efficiency of receiver |
| Minimum detectable power | PIN-TIA | BER measurement at specified data rate | Fundamental receiver sensitivity limit |
Practical Engineering Considerations for Accurate Measurements
Temperature control is critical for both emitters and receivers. Laser diode characteristics are strongly temperature-dependent, with threshold current typically increasing exponentially with temperature. This dependence is characterised by the T0 parameter, with typical values ranging from 50 K to 150 K for InGaAsP lasers. The standard specifies temperature control to within +/-1 degC for most measurements. For fibre-pigtailed devices, coupling efficiency must remain stable throughout measurement. Connector repeatability is a common source of uncertainty in production test environments.
Using integrating spheres for radiant power measurements eliminates errors from beam divergence and spatial non-uniformity. This is particularly important for LEDs where emission patterns vary significantly between devices and with operating conditions such as drive current and temperature.
Frequency-domain measurements require proper impedance matching and calibrated network analysers with de-embedding of test fixture parasitics. The S11 parameter measurement needs careful calibration using short, open, load, and thru standards. The standard explicitly warns against exceeding maximum rated forward current or optical output power, which can cause catastrophic optical damage (COD) to laser diode facets. Current-limited drive circuits with fast over-current protection are recommended for all laser diode measurements.
Exceeding maximum rated forward current or optical output power during testing can cause catastrophic optical damage (COD) to laser diodes, particularly at the facet. Current-limited drive circuits with fast over-current protection are essential for safe and reliable measurements.
Q1: Why is the integrating sphere method preferred for radiant power measurement?
The integrating sphere captures all emitted radiation regardless of angular distribution, eliminating errors from beam divergence and spatial non-uniformity that would affect other measurement approaches.
The integrating sphere captures all emitted radiation regardless of angular distribution, eliminating errors from beam divergence and spatial non-uniformity that would affect other measurement approaches.
Q2: What causes the temperature dependence of laser diode characteristics?
Threshold current increases with temperature due to reduced gain peak alignment with cavity modes and increased non-radiative recombination, characterised by the T0 parameter typically between 50 K and 150 K.
Threshold current increases with temperature due to reduced gain peak alignment with cavity modes and increased non-radiative recombination, characterised by the T0 parameter typically between 50 K and 150 K.
Q3: How does the APD excess noise factor affect receiver performance?
The excess noise factor quantifies additional noise from the statistical avalanche process. A higher factor reduces the SNR improvement from multiplication gain, directly limiting achievable receiver sensitivity.
The excess noise factor quantifies additional noise from the statistical avalanche process. A higher factor reduces the SNR improvement from multiplication gain, directly limiting achievable receiver sensitivity.
Q4: What precautions are needed when measuring pigtailed devices?
Stable fibre positioning is essential as any movement changes coupling efficiency. Connectors need good repeatability, and optical isolators may be needed for laser diodes to prevent feedback-induced measurement errors.
Stable fibre positioning is essential as any movement changes coupling efficiency. Connectors need good repeatability, and optical isolators may be needed for laser diodes to prevent feedback-induced measurement errors.
