AU611523B2

AU611523B2 - Laser chip testing

Info

Publication number: AU611523B2
Application number: AU28514/89A
Authority: AU
Inventors: Albrecht Mozer
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1988-02-01
Filing date: 1989-01-16
Publication date: 1991-06-13
Anticipated expiration: 2009-01-16
Also published as: AU2851489A; DE3802841A1; JPH069283B2; JPH01228189A

Description

611523 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 3) 3 0 3.) 3 (3I~ CiMKErI; SPEC1EFLATCION FOR THE [NVENTPON ENTTLED "LASER CHIP TESTING" 0 o 3) 00 s00C' 3) 3 The following statement Is a Rull description of this Invontion, including the best method of perfonning, it known to us:- Transmitter modules for optical communication contain semiconductor lasers Ps active elements. Hitherto the fabrication of semiconductor lasers had involved the following steps: On a wafer, several hundred semiconductor lasers or laser chips are formed which contain both the active element and a monitoring diode. The wafer is separated into the individual semiconductor lasers or laser chips.

Then, each individual semiconductor laser or laser chip first undergoes a coarse test. Units which have passed this test are individually soldered onto so-called sub-mounts, bonded, and subsequently optically characterized piece by piece. Only those units which meet specific requirements are assembled in laser modules. After the assembly operation, the complete laser 00 0 ooo module is tested to determine whether it meets desired specifications. If o o a the module does not meet the specifications, only limited remedial action can be taken. The optical characterization of the individual chips and the 0 0 laser modules is time-consuming and costly.

From an article by T.L. Paoli, "Blission Properties of Stripe-Geometry Lasers", Applied EHysics Letters 24, page 187 (1974), it is known that the o o intensity of optical noise in semiconductor lasers increases considerably in the region of the threshold current.

From an article by P.A. Andrekso. et al, "Wideband Electrical Noise Measurements for In-Situ Optical Characterization of Laser Diodes", Proo°o ceedings of the 11th European Conference on Optical Communication, Venice, .oood October 1985, pages 733 to 736, it is known that the electrical noise spectrum measured at the input terminals of a semiconductor laser (TFN Terminal Electrical Noise) can be used to characterize a number of optical parameters of the semiconductor laser. For example, one can infer from the noise spectrum whether the laser is operating as a single-mode or multihnode device.

The hitherto used method of characterizing semiconductor lasers has Lhe disadvantage of being very thne-consining and expensive. It is, throefore, the object of the present invention to provide a low-cost method of characterizing semiconductor lasers which can be automated to a large extent. This object is attained by a method as set forth in the main claim.

The method according to the invention has the advantage that no optical measurements are necessary to characterize a semiconductor laser, but purely electrical measurements. From the purely electrical measurements, very precise conclusions can be drawn as to the optical behaviour of the respective laser. The method has an added advantage in that it can also be r used in the assembly of transmitter modules. The optical characteristics of the transmitter modules are optimized on the basis of the electrical noise spectrum. In this manner, laser-light backscatter, for example, can be minimized.

The method and an embodiment of an arrangement for carrying out the o 0 l S method will now be described in detail with reference to the accompanying S dravring, in which: 0 0 0 0 Fig. 1 is a plot of the noise power versus the frequency, the parameo ter being the laser current normalized to the threshold current, o 0 0 Fig. 2 is a plot of the noise power versus the normalized laser cura 0 0 rent, the parameter being the frequency, and 0j Fig. 3 shows an arrangement for measuring the electrical noise spectrum of a semiconductor laser.

o S °0 In Fig. 1, the electrical noise power measured at the terminals of a i 0 0« semiconductor laser is plotted against the frequency. The figure shows three curves designated 1,2, and 3, respectively. The parameter at these curves is the laser current normalized to the threshold current. Curve 1 applies if the laser current is smaller than the threshold current. It can be seen that during operation below the threshold current, the noise power has no distinct maximum, and that above a certain limit, it decreases monotonously with increasing frequency. The reference numeral 2 denotes a curve for which the laser current is equal to the threshold current. Unlike curve 1, curve 2 shows a weak maxlmium. For currents greater than the threshold current (curve the weak maximum becomes a very distinct maximnum. The difference between the noise spectra, depending on whether light emission is stimulated or occurs spontaneously, is conspicuous. It can be used, for example, to precisely determine the laser threshold current.

In Fig. 2, the noise power is plotted against the normalized laser current, the parameter being the frequency. Curve 41 applies for a frequency of 10 MHz, and curve 5 for a frequency of 100 MHz. Both curves show a distinct maximum where the laser current is equal to the threshold current. This penrmits a precise determination of the threshold curt nt.

Fig. 3 is a circuit diagram showing the elements required to measure ca€ o. the noise spectra of Figs. 1 and 2. 6 denotes the semiconductor laser, 7 a spectrum analyzer, 8 a DC source, and 9 a high-frequency bias The o S current source 8 is preferably a very-low-noise current source. It feeds the laser 6, which is mounted on a temperature-stabilized heat sink, for example. The noise spectrum of the current driving the laser is obtained via the radio-frequency bias This spectrum is analyzed in the spectrum analyzer 7. Depending on the plot used, curves as shown in Fig. 1 or 4 4 Fig. 2 are then obtained. The laser 6 of Fig. 3 may be a device on a chip.

Since only electrical and no optical quantities are measured, a decision as to which devices (semiconductor lasers or semiconductor lasers with monitoring diodes) will be used and which will be discarded is made directly on the wafer. A noise spectrum measurrment can be performed on any semiconductor laser, of course.

From the noise spectra, the following laser parameters can be inferred, for example: threshold current quantum efficiency opticalfeedback sensitivity mode spectrum aging behaviour line width.

Claims

1. A method of characterizing optical parameters of semiconductor lasers while said lasers are in a wafer, the method including either: L4br a) measuring the noise power spectra at a fixed frequency while varying the current applied to the laser; or b) measuring the noise power spectra with varying frequency and current ap- plied to the laser.

2. A method as claimd in claim I wherein the current applied to the la- ser is fixed and the frequency is varied.

3. A method as claimed in claim 1 wherein the noise power is measured at fixed frequency and the current applied to the laser is varied.

41. A method as claimed in claim 3 wherein the current applied to the la- ser is varied from values substantially below the threshold current to val- ues above the threshold current. A method as claimed in claim 2 or claim 3 or claim ~4 including the step of deriving the laser threshold current from the measured noise power spectra. 6. A method of characterizing optical p~arameters of semiconductors sub- stantially as herein described with reference to the accompanying drawings. DATED THIS TWENTIETH D)AY OF MACH 1991 ALCATELJ N.V.