JPH069283B2 - A method for characterizing the optical parameters of semiconductor lasers. - Google Patents

Description

The present invention relates to a method for characterizing the optical parameters of a semiconductor laser and an apparatus therefor.

[Prior Art] A transmitter module for optical communication includes a semiconductor laser as an active element. Conventionally, the manufacturing of a semiconductor laser includes the following steps.

Hundreds of semiconductor lasers or laser chips are formed on a wafer, which include both active devices and monitoring diodes. The wafer is divided into individual semiconductor lasers or laser chips. Then each individual semiconductor laser or laser chip is first subjected to a rough test. Each unit that passes this test is soldered and bonded to a so-called submount and is optically characterized one by one. Only units that meet specific requirements are assembled into a laser module. After assembly, the completed laser module is tested to determine if it meets the desired specifications. If the module does not meet specifications, only limited corrective action will be taken. Optical characterization of individual chips and laser modules is time consuming and adds cost.

References (Applied Physics Letters 24, 187 pages, 1974
Et al.) That the intensity of optical noise in a semiconductor laser increases significantly in the region of threshold current.

Literature (Proceedings of the 11th European Conference
on Optical Communication, Venice, October 1985, pages 733 to 736), in which the electrical noise spectrum (TEN = terminal electrical noise) measured at the input terminal of a semiconductor laser defines the characteristics of many optical parameters of the semiconductor laser. It can be used for. For example, it can be estimated from the noise spectrum whether the laser is operating as a single mode device or a multimode device.

[Problems to be Solved by the Invention] The conventionally used methods for characterizing semiconductor lasers have the drawback of being very time consuming and expensive. Therefore,
It is an object of this invention to provide a low cost method of characterizing a semiconductor laser that can be highly automated.

This object is achieved according to the invention in that the spectrum is measured at the semiconductor laser electrical input with the semiconductor laser also on the wafer.

The method of the invention has the advantage that no optical measurements are required to characterize the semiconductor laser, only purely electrical measurements are necessary. Being a purely electrical measurement, very accurate conclusions can be drawn about the optical properties of each laser. This method has the additional advantage of being applicable to the assembly of transmitter modules. The optical characteristics of the transmitter module are optimized based on the electrical noise spectrum. In this way, for example, backscattering of laser light can be minimized.

[Embodiment] An embodiment will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a graph showing the relationship between the electric noise power measured at the terminals of a semiconductor laser and the frequency. There are three curves shown as 1, 2, 3 respectively. The parameter in these curves is the laser current normalized to the threshold current. Curve 1 applies when the laser current is less than the threshold current. At operating conditions below the threshold current, the noise power does not have a clear maximum and above a certain limit decreases monotonically with increasing frequency. Reference numeral 2 shows a curve when the laser current is equal to the threshold current. Unlike curve 1, curve 2 exhibits a weak maximum. At currents above the threshold current (curve 3), this weak maximum changes to a very distinct maximum. The difference between such noise spectra depending on whether the light emission is stimulated or spontaneous is significant. This can be used, for example, to accurately determine the laser threshold.

In FIG. 2, the noise power with respect to the normalized laser current is shown. The parameter is frequency. Curve 4
Curve 10 is for 100 MHz and curve 5 is for 100 MHz. Both curves show different maxima at the position where the laser current is equal to the threshold current. This allows an accurate determination of the threshold current.

FIG. 3 is a circuit diagram of an apparatus required for measuring the noise spectrum of FIGS. 1 and 2. In the figure, 6 is a semiconductor laser, 7 is a spectrum analyzer, 8 is a DC power supply, and 9 is
Is the high frequency bias "T". The DC power supply 8 is preferably a very low noise current source. It powers, for example, a semiconductor laser 6 mounted on a temperature-stabilized heat sink. The noise spectrum of the current driving the laser is obtained via the high frequency bias "T". This spectrum is analyzed by the spectrum analyzer 7. Depending on the use, a curve like that shown in FIG. 1 or 2 is obtained. The laser 6 of FIG. 3 may be an on-chip device. Since only electrical quantities are measured, not optical quantities, it is possible to directly determine on the wafer whether the device (semiconductor laser or semiconductor laser with monitor diode) can be used or should be discarded. Can be done. The measurement of the noise spectrum can of course be performed on any semiconductor laser.

The following laser parameters can be estimated from the noise spectrum.

* Threshold current * Quantum efficiency * Optical feedback sensitivity * Mode spectrum * Relaxation frequency * Aging characteristics * Line width

[Brief description of drawings]

FIG. 1 shows the characteristics of noise power vs. frequency, and the parameter is the laser current normalized by the threshold current. FIG. 2 is a characteristic of the noise power versus the normalized laser current, and the parameter is the frequency. FIG. 3 shows an apparatus for measuring the electrical noise spectrum of a semiconductor laser. 6 ... Semiconductor laser, 7 ... Spectrum analyzer, 8 ... DC power supply, 9 ... High frequency bias.

Claims (10)

[Claims] 1. A method for characterizing optical parameters of a semiconductor laser based on an evaluation of the electrical noise spectrum, the electrical noise spectrum being measured at an electrical input terminal of the semiconductor laser while the semiconductor laser is still on a wafer. A method for determining characteristics of optical parameters of a semiconductor laser, which is characterized by the above. 2. Method according to claim 1, characterized in that the electrical noise spectrum is measured by a spectrum analyzer. 3. A method according to claim 1 or 2, characterized in that the electrical noise spectrum is measured at a constant frequency and at a variable laser current. 4. A method according to claim 3, characterized in that the laser current is varied from a value well below the threshold current to a value above the threshold current. 5. A method according to claim 1 or 2, characterized in that the electrical noise spectrum is measured at a constant laser current and a variable frequency. 6. The method according to claim 1, wherein the presence of a single mode laser is estimated from the electrical noise spectrum. 7. One or more of the following parameters characterizing the laser: threshold current, quantum efficiency, mode spectrum, relaxation frequency, and line width, are estimated from the electrical noise spectrum. The method according to any one of claims 1 to 6, which comprises: 8. A method according to claim 1, wherein the aging characteristics of the laser are estimated from the electrical noise spectrum. 9. A method for characterizing optical parameters of a semiconductor laser based on an evaluation of the electrical noise spectrum, characterized in that the electrical noise spectrum is measured at an input terminal of the semiconductor laser module. A method of defining the characteristics of parameters. 10. Method according to claim 9, characterized in that the optical properties of the module are optimized on the basis of the electrical noise spectrum.