## 1 Introduction  
Due to the advantages of a high resolution, high accuracy and direct traceability to the primary standard of length, laser interferometers have been widely applied for measuring the displacement or geometric errors of precision machines or measurement equipment [1; 2; 3].  
A He-Ne laser with a stabilized frequency is generally adopted as the laser source of the laser interferometers. However, the He-Ne lasers are too voluminous, and He-Ne laser-based laser interferometers are usually bulky. Therefore, He-Ne laser-based laser interferometers can only be used as off-line measurement instruments. There is an increasing requirement for on-line measurement. Therefore, recently, lots of research efforts have been directed towards the development of miniature laser interferometers, which can be embedded in precision machines or measurement equipment as an on-line measurement sensor [4; 5; 6; 7; 8]. A laser diode (LD), which has the advantages of a small size, a long-operating time and a high-output power, is widely adopted as the laser source of miniature laser interferometers [4; 5; 6; 7; 8; 9; 10]. However, the LD suffers from low stability and low accuracy in terms of wavelength. Since the wavelength is the length unit of the laser interferometer, it is essential to improve the stability and the accuracy of the LD wavelength in order to achieve a high-precision and stable displacement measurement.  
There are many approaches and techniques for stabilizing the laser wavelength/ frequency [11; 12; 13; 14; 15; 16; 17; 18]. Efforts aim to stabilize the frequency of LDs by locking them to the reference frequency of atoms or molecules, in which the LD's frequency is modulated and turned around the frequency transition of the reference frequency of atoms or molecules [11; 12].  
The laser frequency can also be stabilized to the resonances of a Fabry-Perot cavity or a photonic crystal cavity [13; 14; 15]. The stabilization of laser frequency can also be carried out based on a high-accuracy wavelength meter [16; 17; 18]. However, due to the complexity of the construction, the above-mentioned methods cannot be applied for stabilizing the LD wavelength/frequency in miniature laser interferometers.  
In addition, the incomplete separation of the differently polarized partial beams in the interferometer as well as multiple back reflections will cause the optical mixing. Inevitable nonlinear errors, which are primary caused by the optical mixing, will thus restrict the performance of the miniature laser interferometers. A group of studies have been carried out to eliminate the nonlinear errors, such as the ellipse-fitting method [19], the recursive weighted least-squares method combined with the Kalman filter algorithm [20], the optimization of the optical path structure [21], the field programmable gate array [22] and the intermodulation [23]. Although the above-mentioned methods can effectively correct nonlinear errors, they involve time-consuming calculation or a complicated hardware system.  
Therefore, methods for improving the measurement accuracy of a miniature laser diode interferometer (MLDI), in which a laser diode is adopted as the laser source, are proposed in this paper. The nonlinear errors of the MLDI induced by the installation errors and the manufacturing errors of the optical elements are investigated. The error sensitivity analysis of the nonlinear errors is analyzed. In order to stabilize the LD wavelength with high stability for a long period of time, an active wavelength stabilization method based on a real-time laser wavelength meter is proposed.