WAVELENGTH SCANNING INTERFEROMETRY AND INTERFEROMETER EMPLOYING LASER DIODE

Abstract: In a laser diode heterodyne interferometry, a light beam from a laser diode (1) is divided in two, one of which is used as a reference beam and the other of which is used as an inspecting beam, and the two beams are projected again on an identical plane so as to form an interference pattern. The injection current of the laser diode (1) is mod­ ulated to scan the interference pattern, and a photocurrent obtained from each element of a photosensor array (6) which receives an intensity variation of the interference pattern is divided by a monitored current of a light output of the laser diode (1), to obtain a photocurrent signal of nor­ malized interference pattern intensity variation.

Description: Background of the Invention: The present invention relates to an optical interferometry and interferometer utilizing the interference phenomenon of laser beams. More particularly, it relates to a wavelength scanning interferometry and interferometer with a laser diode that are well-suited for application to, e. g., the inspection of optical elements of which a high precision is required for reading an interference pattern. In a prior-art optical interferometer, the wavelength of a light source is fixed, and such a method as, for example, providing one of beams with a frequency shifter or modulating the phase of light by means of a piezoelectric device is employed for raising the reading precision of an interference pattern. However, both the devices are expensive. Other disadvantages are that a large number of portions must be adjusted and that a driving power source of high voltage is required. An interferometer of this type is described in EP-A-0 082 050 for example. More specifically, in an optical interferometer based on the so-called heterodyne interference method in a prior art, the wavelength of a light source is fixed. In order to measure also the phase of the wavefront between interference fringes, a method is employed in which the wavelength of one of the beams is shifted by a frequency modulator or in which a piezoelectric device is mounted on a mirror so as to perform phase modulation. Both the methods, however, are disadvantageous in that the modulation devices are expensive and require high voltages. The latter method involves complicated handling for the corrections of nonlinearity and hysteresis, etc. The variation of the wavelength emitted by a laser diode by modulating the diode current has been used in the prior art for other purposes than wavelength scanning interferometry. US-P-3 805 074 for example describes an apparatus for monitoring air pollution sources. In this apparatus the wavelength of the laser light is scanned through an absorption line of a pollutant gas. In Electronics Letters, Vol 21., No. 3, January 31, 1985, a displacement measurement system using an interferometer method is described. In this method the optical path difference of an interferometer is measured by determining the frequence deviation of a laser diode required to drive the phase of the interferometer output through one full cycle. These known systems are not suited for inspection of optical elements. Summary of the Invention: An object of the present invention is to eliminate the disadvantages described above, and to provide a laser diode interferometer which is inexpensive and simple and is easy of manipulation and which permits to electrically read an interference pattern and to perform interference pattern processing at high precision. The present invention consists in that a laser diode is employed as the light source of an interferometer of the Fizeau type, the Michelson type, the Twyman-Green type, the Mach-Zehnder type or the like and that the drive current of the laser diode is modulated to scan the wavelength thereof, thereby to scan an interference pattern and to enhance the reading precision of the interference pattern. That is, with note taken of the unstable characteristic of a laser diode that the wavelength thereof changes delicately depending upon the amount of current applied, the present invention consists in that the wavelength of the laser diode itself is changed by utilizing the characteristic contrariwise. According to the present invention, the whole apparatus is miniaturized. The accuracy can be raised double or more, and the required power can be lowered to one thousandth. Moreover, a ruggedness of as minute a level as hundred thousandth mm can be measured. The interference pattern is formed on a two-dimensional photodetector array. The present invention can measure a lens, a mirror etc. and also the front of the light wave of a laser source, and is extensively applicable to the design of laser devices, etc. As another feature of the present invention, the quantity of light having fluctuated due to the change of the drive current of a laser diode is monitored, and the fluctuation of the intensity distribution of an interference pattern is normalized, to perform electric processing of higher precision. Brief Description of the Drawings: Fig. 1 is a graph showing the amount of current change which is required for shifting the wavelength of a laser diode by one wavelength, versus the difference of the optical lengths of the two light beams of an interferometer; Fig. 2 is a graph showing the current - light output characteristic of a laser diode; Fig. 3 is a diagram showing the change of the injection current of a laser diode; Fig. 4 is a diagram showing the change of an interference pattern; Fig. 5 is a graph showing the relationship between the difference of optical lengths and the amount of injection current change; Fig. 6 is a diagram of the application of the present invention to a Mach-Zehnder interferometer; Fig. 7 is a diagram of the application of the present invention to a Twyman-Green interferometer; Fig. 8 is a diagram showing an example of an interference pattern according to the present invention; Fig. 9(a) is a circuit diagram showing an example of a laser diode driver circuit for use in the present invention, while Fig. 9(b) is a signal waveform diagram thereof; and Fig. 10 is a diagram showing an example of a divider circuit. Description of the Preferred Embodiments: First, the principle of the present invention will be described. In general, in a laser diode, when the drive current is changed by ΔI (mA), the wavelength shifts. For example, in case of a channeled substrate planar type laser diode, the shift magnitude Δλ of the wavelength is given by: Δλ = 0.006·ΔI (nm) (1) On the other hand, the complex amplitude of light at an arbitrary point (x, y) on an interference pattern is given on the time average by: Here, u₀ and u r denote the amplitudes of an object beam and a reference beam respectively, r₀ and r r denote the optical lengths of the object beam and the reference beam from a reference plane respectively, and k denotes a wave number which is given by: k = 2π λ From the above, the intensity distribution is: Accordingly, an interference pattern is obtained which has the maximum intensity at a point (x, y) satisfying 2π / λ {r₀(x, y) - r r (x, y)} =2mπ (m: integer) and the minimum intensity at a point satisfying 2π / λ {r₀(x, y) - r r (x, y)} = (2m+1)π (m: integer) That is, the intensity distribution I(x, y) is: I(x, y) = I₀ [1 + γ cos {Φ(x, y)}] (2) Here, I₀ = u₀²(x, y) + u r ²(x, y) γ = 2 u₀·u r /{u₀²(x, y) + u r ²(X, y)} Φ(x, y) = 2π λ (r₀ - r r ) = 2π λ ·ℓ Φ(x, y) is the phase of a wavefront to be measured, and ℓ is the optical path difference at the point (x, y). Now, let's consider to scan the interference pattern by one wavelength (2 π phase). Assuming that there be n waves within the difference of the optical lengths of the two beams, the difference being: ℓ = r₀(x, y) - r r (x, y) the following holds: n·λ = ℓ (3) Meanwhile, as stated before, the wavelength λ of the laser diode fluctuates when the injection current i is changed. Now, when the injection current is changed linearly with time (Δi = α·t), the wavelength slightly shifts (Δλ) as: Δλ = β·Δi = β·α·t (α, β: constants) (4) (β ≃ 0.006). When the drive current I of the laser diode is increased to shift the wavelength by Δλ in the direction of a longer wavelength, (n - 1) waves exist within the optical path difference ℓ, and hence, the following holds: (n - 1)(λ + Δλ) = ℓ (5) When n is eliminated from Eqs. (3) and (5), is obtained. Using a differential coefficient, the above will be explained more. As to the very small change Δλ of the wavelength, the phase Φ of the foregoing wavefront can be written as: λ₀ denotes the initial wavelength. Here, the phase shift ΔΦ is: ΔΦ = (2π·ℓ/λ ₀ ²)Δλ (8) and Eq. (6) is given for a shift of 2 π. From Eqs. (1) and (6), Δi = 100/ℓ (mA) (9) Eq. (9) gives the current change Δi for shifting the interference pattern by one fringe in the interferometer of the optical length difference ℓ. Fig. 1 illustrates this relationship graphically. For example, when the difference of optical lengths is 10 cm, the current change amount becomes Δi = 1 mA. The operating median of a diode laser is usually 60 - 100 mA as illustrated in Fig. 2. It is therefore understood that the wavelength scanning is possible with a very slight current modulation. In addition, the change of the light output is very slight for such a current change. This will be repeatedly explained. When the injection current i of a laser diode 1 is changed as shown in Fig. 3, the interference pattern starts reciprocative scanning as indicated by arrows in Fig. 4. It is understood from Eq. (7) that the amount of change of the injection current required for shifting the interference pattern by one fringe depends upon, the difference of optical lengths. More specifically, when putting ΔΦ = 2π in Eq. (8), holds. This value is the current change amount affording the one-fringe shift in the case of a single-path interferometer, and ½ thereof is the change amount in the case of a double-path interferometer. These relationships are illustrated in Fig. 5, and the constant β is experimentally found. When the optical , path difference is ℓ = 100 mm, the injection current change amount is Δi = 1.0 mA, and the interference scanning of, one fringe is possible. Needless to say, half of the above suffices for the interferometer of the double path type. Back to the original equation (7), mathematical expressions will be traced. In combination with Eq. (7), the very small change ΔΦ of the phase is: This affords the basic signal of the heterodyne interferometry. Here, the following holds: f = (2π·ℓ/λ₀²)β·α (11) Let's consider to receive and photoelectrically convert the interference pattern by means of a two-dimensional photodiode array such as CCD (charge coupled device). Since the intensity change of the interference pattern on one element of the CCD is given by Eqs. (2), (7) and (10), the photocurrent I p is given by: I p (x, y, t) = I₀ [1 + γ cos[Φ(x, y, t)}] (12) Φ(x, y, t) = ₀(x, y) - 2π f t This is called the 'heterodyne signal' As understood from Eq. (10), when the difference ℓ of the optical lengths of the reference beam and the object beam is zero, the heterodyne signal is not obtained even by changing the injection current i . In addition, when the optical length difference ℓ is small, a sufficient phase change is not obtained unless the change of the injection current is enlarged. However, when the injection current is greatly changed, the light output itself of the laser diode changes, so that I₀ changes in Eq. (2), making it impossible to obtain the original heterodyne signal (12). Therefore, the second point of the present invention consists in obtaining the normalized interference pattern intensity distribution as in the following equation: by monitoring the quantity of light I₀ of the laser diode and dividing the photocurrent I p by the monitored current. That is, dividers are connected to the respective elements of the CCD. Fig. 6 is a setup diagram showing an embodiment of the present invention, and illustrates the case of a Mach-Zehnder type interferometer. This interferometer employs a laser diode 1 as a light source. A light beam from this diode laser is turned by a condenser lens 2 into a collimated beam, which is split into two beams by a beam splitter 3-1. One of the beams is used as a beam for inspection, in which a transmissive object 10 to be measured is placed and which is reflected by a reflector mirror 4-2 and is combined via a second beam splitter 3-2 with the other beam (reference beam) from a reflector mirror 4-1 to form an interference pattern on a photodetector 6 of a two-dimensional photosensor array (CCD). The difference between the optical lengths of the two beams can be adjusted by rotating the beam splitter 3-1 and moving the reflector mirror 4-1. Fig. 7 is a setup diagram showing another embodiment of the present invention, in which the invention is applied to a Twyman-Green type interferometer. A beam from a light source constructed of a laser diode 1 is turned by a condenser lens 2 into a colliminated beam, which is split into two beams by a beam splitter 3. One of the beams to serve as a reference beam is reflected by a reflector mirror 4, and is transmitted through the beam splitter 3 to fall on a photodetector (CCD) 6. The beam transmitted through the beam splitter 3 is employed as an inspection beam This beam is reflected by an object to-be-inspected 10 and is further reflected by the beam splitter 3, to form an interference pattern on the photodetector (CCD) 6 conjointly with the reference beam. At this time, the difference of the optical lengths of the two beams can be adjusted by moving the reflector mirror 4 in the direction of the optic axis thereof. Fig. 8 is a diagram showing an example of the interference pattern obtained with this interferometer. In the figure, the spacing between interference fringes L₁ (indicated by a solid line) and L₂ (indicated by a dotted line) corresponds to the shift of a wavefront equal to one wavelength. In case of changing the wavelength of the laser diode 1 from λ₁ to λ₂, the interference fringe L₁ appears for the wavelength λ₁ and that L₂ appears for the wavelength λ₂, so that the interference fringe moves. When the current of the laser diode 1 is modulated by a triangular wave of frequency ω and amplitude Δi as shown in Fig. 3 and is photoelectrically converted by the photodetector 6 so as to obtain data, the resulting signal is passed through a filter which has a pass frequency f given by Eq. (11), whereby only the frequency component can be extracted as a signal of high signal-to-noise ratio. That is, an oscillating component attributed to the disturbance of the interferometer and the influences of temperature etc. can be eliminated, and the reading precision can be enhanced by repeating measurements a large number off times and irregularly and taking an average. Further, a phase distribution after the error and inclination of the interferometer itself, the error of focusing, etc. have been subtracted can be displayed by a contour map or the like. Fig. 9(a) is a circuit diagram showing an embodiment of a laser diode driver circuit for use in the present invention, while Fig. 9(b) is a signal waveform diagram thereof. This circuit serves to modulate the semiconductor laser or laser didode LD (corresponding to the laser 1 in Figs. 6 and 7) with an amplitude Δi having a center value at a bias current i₀ . Now, when a voltage of -V E volts is applied to a point A in the figure and a potential at a point B (the base of a transistor Q) is set to -V B volts by a variable resistor R₁, the following voltage is applied across the laser diode LD: V = (-V B - 0.7) - (- V E ) Consequently, a current i₀ = V(R₃ + r) where r denotes the resistance of the laser diode can be caused to flow through the D.C. resistance R₃, and the laser diode LD oscillates. The current of this value is used as the bias current. The current modulation whose amplitude is Δi is performed in such a way that a triangular wave of frequency ω as shown in Fig. 9(b) is externally input to a point C in the figure, and that the laser diode is driven by this modulating current. In this regard, as shown in Fig. 2, the light output of the laser diode changes when the injection current changes. Therefore, the light output of the laser diode 1 is monitored by means of a photodetector 8 disposed in a laser module, and the interference pattern intensity is normalized with the output thereof. A practical divider circuit therefor is shown in Fig. 10. A photocurrent based on the interference pattern intensity (the output of each element of the CCD 6) is input to the numerator Z of a divider commercially available, AD532 (manufactured by Analog Device Inc.), while a photocurrent derived from a light output behind the laser diode (the output of the photodetector 8) is input to the denominator X, thereby to perform the following division: Z/X With this method, the normalized interference pattern signal (Eq. (13)) is obtained. More in detail, a signal from any element 6-1 of the CCD is input to the input terminal Z of the divider AD532 through an amplifier 11. On the other hand, the D.C. output lights of the laser diode is received by the photodetector 8 and is input to the input terminal X of the divider 14 via two stages of amplifiers 12 and 13. As a result, an output signal Out is obtained: When this signal is multiplied by -10, Z/X namely, in Eq. (13) is obtained. As described above, according to the present invention, the reading of an interference pattern at high precision is permitted merely by modulating the drive current of a laser-diode light source slightly. Further, it becomes possible to perform at high precision the measurement of the wavefront of the spot of an optical head for use in the recording and reproduction of an optical disc and the measurement of an aberration inherent in the laser diode itself. The above description has clarified that the interference scanning can be carried out without a piezoelectric device or a frequency shifter. When the heterodyne signal obtained with Eq. (12) is applied to, for example, a signal processing method based on the integrated bucket technique developed by J. C. Wyant et al., it becomes possible to electrically evaluate the wavefront phase Φ(x, y) and to construct an interferometer system which can input data to a computer and which is higher in precision and more advantageous for data processing.

Claims: A wavelength scanning type laser diode interferometer having a laser diode (1), a driver circuit (Fig. 9) for supplying the laser diode with a drive current (I) the amplitude of which changes periodically and linearly, photodetector means (8) for monitoring the quantity of light output of the laser diode (1), beam splitting means (3; 3-1) for splitting a light beam from the laser diode (1) into a reference beam and an inspecting beam, beam combination means (3; 3-2) for combining the reference beam and the inspecting beam into a combined beam forming an interference pattern on a two-dimensional photodiode array (6; 6-1) and divider means (14) for dividing respective outputs (Ip) of the two-dimensional photodiode array by the output (Io) from the photodetector means to obtain a photocurrent signal (Ip') of normalized interference pattern intensity change. A wavelength scanning type laser diode interferometer according to claim 1, wherein the two-dimensional photodiode array (6; 6-1) comprises a plurality of photo-elements for detecting simultaneously the intensity at many points of the interference pattern formed on the two-dimensional photodiode array (6; 6-1) and for outputting a photocurrent corresponding to respective intensity. A wavelength scanning type laser diode interferometer according to claim 1 or 2, wherein the wavelength of the laser diode (1) is periodically changed by a value which shifts the interference pattern by one interference fringe. A method of measuring an interference pattern formed by a wavelength scanning type laser diode interferometer comprising the steps of: monitoring the light intensity emitted by a laser diode (1); splitting a light beam from the laser diode into a reference beam and an inspecting beam; combining the reference beam and the inspecting beam into a combined beam; projecting the combined beam onto a two-dimensional photodiode array (6; 6-1) so as to form an interference pattern; detecting simultaneously light intensity of the interference pattern at a plurality of positions on the two-dimensional photodiode array (6; 6-1); and dividing the respective light intensity of the interference pattern by the monitored light intensity, so as to normalize the interference pattern formed on the two-dimensional photodiode array (6; 6-1).

Inventors: KATAOKA, KEIJI, OHTAKE, MASATOSHI, TATSUNO, KIMIO, TSUNODA, YOSHITO, YONEZAWA, SEIJI
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: G01J9
IPC codes: G01B9, G01J9, G01J3