The laser interferometer with a wavelength of 632 nm is the length benchmark of today's nano age and is also the precision guarantee of advanced manufacturing (machine tools, lithography machines, aerospace, etc.). The spectral line of the helium-neon (He-Ne) laser with a wavelength of 632 nm is narrow and has a natural highly stable frequency point that can be regarded as the mark point of the high stability of frequency (wavelength). The light with a wavelength of 632 nm is orange-red, which can facilitate the alignment of light paths. These excellent characteristics of the He-Ne laser make it the best choice as a light source for single-frequency laser interferometers and dual-frequency laser interferometers. Most lithography machines choose dual-frequency laser interferometers to guarantee nanoscale measurement accuracy. Due to their large size and limited lifetime, numerous research was conducted to replace He-Ne lasers with semiconductor lasers but failed.

Due to the limited lifetime of He-Ne lasers, interferometers used in lithography and other applications often need to be stopped to replace the end-of-life lasers. However, the following are the real bottlenecks confronted, which need to be studied preferentially. First, we used to transform single-frequency lasers into dual-frequency lasers by the Zeeman effect (Figs. 1-2). One drawback of Zeeman dual-frequency lasers is that a large frequency difference and high power cannot be achieved simultaneously; in other words, if the frequency difference is large, the power becomes small, which cannot meet the larger frequency difference (such as 10, 20, and 40 MHz) requirement of lithography machines. Second, as early as 1983, it was discovered that either single-frequency or dual-frequency laser interferometers have a nonlinear error as large as a few nanometers or even dozens of nanometers. This error has long been found by foreign measurement units and National Institute of Metrology of China, but it has not been solved.

In this regard, China faces the problem that it cannot manufacture the He-Ne lasers of the kovar-glass assembly structure for laser interferometers. Such He-Ne lasers purchased in the market have a high elimination rate due to frequency instability and mode jumps.

To break through the two technical bottlenecks of Zeeman dual-frequency lasers, our research team started the study of the birefringent dual-frequency laser. In 1985, we placed a crystal quartz plate inside a He-Ne laser. The birefringence of crystal quartz caused the laser frequency to split, and one frequency was divided into two frequencies with orthogonal polarization. Subsequently, the frequency splitting is caused by the stress birefringence inside the optical glass plate in the laser, and then the stress birefringence cavity mirror causes the laser to produce two frequencies. By the stress birefringent cavity mirror, the surface of the mirror substrate facing the laser gain tube is coated with an anti-reflection film so that the laser beam passes through without loss. The other surface of the mirror substrate is coated with a laser reflection film as the laser cavity mirror (Figs. 4-6). The stress in the optical substrate causes the laser to change from single frequency to double frequencies. This laser can emit a frequency difference of more than 40 MHz, but it can achieve a frequency difference of less than 40 MHz.

The frequency difference from 1 MHz to 40 MHz is the most useful interval for dual-frequency laser interferometers as the Doppler frequency shift caused by the velocity of the measured target displacement is in this region for most dual-frequency laser interferometers. If the laser frequency difference is large, the electronic system matching with the laser will be complicated and costly, which is unnecessary for some applications.

The experiments show that when the difference between two frequencies is less than 40 MHz, the coexistence width of the two frequencies is zero; with one extinguished, the frequency difference disappears. The reason why the frequency difference of less than 40 MHz cannot be obtained is theoretically analyzed, namely that the intense mode competition of the laser makes one of the two frequencies extinguished. The scalar Lamb theory overestimates the competition intensity between modes while the Lamb ring laser theory underestimates the inhibitory effect of competition on one of the two frequencies, which is inconsistent with our experiments. We choose the extended Lamb theory and consider the effects caused by the orthogonal polarization characteristics of laser beams, the degeneracy of the atomic energy level, the ratio of two isotopes of Ne, the collision between atoms, and other factors. The theoretical analysis results are consistent with the experimental results. It is shown that when the frequency difference is less than 40 MHz, one of the two frequencies is always in the suppressed state and alternates between winning and being extinguished due to the change of the cavity length (Fig. 3). The relationship between the width of the coexistent frequency domain of the two frequencies and the laser frequency difference is calculated (Fig. 7).

Furthermore, our team looks for a method to generate a frequency difference less than 40 MHz. First, the frequency of the laser is split by birefringence; as a result, a laser beam contains two optical components with perpendicular (or orthogonal) polarization to each other. Then, a transverse magnetic field is applied to the laser, and the transverse Zeeman effect divides the gain atoms into two groups. Thus, each one of the two polarized lights obtains a gain from its atomic group, and there is no competition between them so that the two frequencies with a difference of less than 40 MHz can oscillate independently. The main direction of birefringence and the transverse magnetic field direction should be consistent.

We put forward the concept of "internal engraving stress" for generating dual frequencies. The narrow-pulse laser is focused on the inside of the cavity mirror substrate to engrave the stress birefringence of the He-Ne laser (Fig. 8). The "internal engraving stress" improves the precision of the frequency difference. The polarization of light at two frequencies is perpendicular to each other, which can be explained by the photo-elastic theory. A birefringence-Zeeman dual-frequency laser is made by adding the transverse magnetic field to the internal engraving stress birefringence dual-frequency laser. In a wide range of applications, this laser has proved its excellent characteristics.

The internal engraving stress dual frequencies need to be implemented directly for the He-Ne laser. Hence, a dual-frequency He-Ne laser with kovar-glass structure is developed, which has the power of about 1 mW and a length of about 150 mm (Fig. 9). At the same time, it solves the problem that China was unable to manufacture He-Ne lasers of kovar-glass structure, as well as the problem of the high elimination rate of He-Ne lasers in the manufacturing of laser interferometers.

The birefringent dual-frequency lasers with kovar-glass assembly structure (non-blowing technology) and interferometers have been mass-produced, and so are the temperature, humidity, pressure sensors, and calibration systems, which meet the needs of scientific research and the industry. The dual-frequency laser interferometer is tested by National Academy of Metrology of China, and the results are as follows: the frequency stability is 10^-8-10^-9; the resolution is 1 nm; the nonlinear error is less than 1 nm, and the length measurement error of 70 m is less than 5 μm.

This paper introduces the whole-chain technology of dual-frequency laser interferometers completed by our research team, including the laser with kovar-glass assembly structure→internal engraving stress birefringence dual-frequency laser→birefringent dual-frequency laser interferometer. We solve the two bottlenecks of the traditional Zeeman dual-frequency laser interferometry, namely, the nonlinear error of measurement as large as a few nanometers or even more than 10 nm and the impossible coexistence of a large frequency difference and high power. The laser power is 1 mW, and the frequency difference ranges from 1 MHz to hundreds of MHz, with a nonlinear error of less than 1 nm. Upon the replacement of the failed laser for a lithography machine, the positioning error of the workbench is reduced to a quarter of what it was.

Our team has carried out research on the solid-state micro-chip dual-frequency laser interferometer, which is smaller in size (e.g., 3 mm×3 mm×1 mm) and consumes less power (less than 1 W) than the He-Ne laser interferometer and can achieve nanoscale resolution. Despite its wide application scope, its accuracy is not comparable to that of the He-Ne laser interferometer. It seems that the He-Ne dual-frequency laser would remain the main source of the interferometer for a long period of time, but it is hoped that one day the solid-state micro-chip dual-frequency laser interferometer will have the same accuracy as the He-Ne dual-frequency laser interferometer.