This invention related generally to systems used to characterize materials through optical analysis and, in particular, to an integrated, external cavity diode laser module applicable to Raman, fluorescence and other forms of stimulated emission.
In Raman spectroscopy, coherent light directed on a sample is scattered at a number of discrete frequencies above and below that of the incident radiation. The wavelength shifts of the Raman lines, their intensity, and their polarization are characteristics of the scattering substance. The net effect is that the wavelengths of the Raman spectrum for any given material will have a predetermined difference from the wavelength of the incident light. That is, if a Raman spectrum is measured with one wavelength of incident light, and then measured a second time using a different wavelength of incident light, the same Raman wavelength line pattern will be measured, but shifted in wavelength. The number of Raman lines and their wavelength shifts from the incident wavelength remain constant. Raman scattering is similar to fluorescence except for the nature of the energy-level transitions involved.
Raman spectroscopy has proven to be very useful in characterizing the molecular content of unknown materials in chemical, medical, and other industrial and academic applications. A typical high sensitivity Raman system includes a laser which is focused on a sample cell. The Raman scattered light is collected by a lens, polarized, filtered, focused and dispersed by one or more gratings to separate the light into its characteristic spectrum.
The high-cost items in this typical package include the laser (i.e., argon-ion or helium-neon), the collection lens, and certain components associated with detection. All devices must exhibit the sensitivity required to measure Raman line intensities which are typically several orders or magnitude below that of the incident light.
To lower system cost and size, it would be advantageous to use a laser diode as opposed to a more expensive gas laser. Laser diodes with adequate power output are now available, however, they tend to oscillate in several modes (frequencies) and are therefore unsuitable for Raman spectroscopy. FIG. 2A, for example, shows the output of a free-running, 1-Watt, 808-nm diode laser at temperatures and times without stabilization.
In a typical laser diode laser, frequency is entirely dependent on the band gap which, in turn, is dependent on injection current and the temperature of the device. Changes to either often lead to changes in wavelength. To stabilize temperature in spectroscopic applications, a diode laser is therefore used with a thermoelectric cooler. However, the change in band gap with temperature is often subject to hysteresis, such that reproducing a set of operating conditions does not necessarily reproduce the same wavelength. Under such conditions, acquired Raman spectra will exhibit an apparent shift when compared to previously acquired spectra at the same temperature.
Another problem is that spurious radiation from the spectrographic system may be reflected back into the laser diode, which can result in xe2x80x9cmode hopping.xe2x80x9d in which the main laser output frequency hops from one mode to another. Particularly as the current level is changed to increase or decrease the optical power output, regions of instability are encountered causing the laser wavelength to shift, emit multiple wavelengths, or even oscillate between wavelengths in an uncontrolled manner. xe2x80x9cMode hopsxe2x80x9d can also be caused by optical feedback from various optical elements comprising the system, including collimating optics, pigtails, and so forth. Again, even if a region of stability is found, there is no guarantee that the device will remain stable over time.
Given these circumstances, laser diodes must be stabilized to suppress side-mode oscillation and mode hopping before they can be used in certain industrial applications, including spectroscopy. A recent improvement is the use of an external cavity in conjunction with a diode laser. In such an arrangement, an external grating is used to provide selective feedback to the active region so that lasing only occurs at one wavelength. Unwanted wavelengths are dispersed outside of the cavity by the grating. The selective feedback also eliminates hysteresis and mode hopping.
This invention resides in an external cavity diode laser system. Although the invention finds utility in many application areas, the package is intended and optimized for use in fiber-coupled Raman Spectroscopy.
The laser diode is coupled to an optical grating forming an external cavity configuration outputting a beam of light having a nominal wavelength along an optical path. Important to the invention, a holographic bandpass filter is disposed in the optical path to transmit elements of the beam having the nominal wavelength and reject beam elements of differing wavelengths.
The bandpass filter includes a transmission grating having first and second sides supported so that the beam intersects the first side at an angle thereto so as to disperse the various wavelengths of the beam through angles which are a function of their wavelength, and one or more optical elements supported in relation to the second side of the holographic transmission grating for forming an exit beam of narrowband light at the nominal wavelength while rejecting dispersed light of a wavelength differing from the nominal wavelength. In a fiber-coupled arrangement, a lens is used for focusing the exit beam from the module onto the entrance of the fiber. In this configuration, the combination of the transmission grating and the focusing of the exit beam operates as the bandpass filter.
The laser diode has a long dimension and, in the preferred embodiment, the transmission grating is oriented such that the elements of differing wavelengths are dispersed perpendicular to the long dimension of the diode. The module may further include a half-wave retarder supported between the external cavity and transmission gratings to rotate TM polarization orientations to TE polarization orientations.
An optical isolator may be disposed in the optical path prior to the exit beam to control spurious reflection, and an optical sensor, preferably supported to receive weak reflections from the transmission grating, may be used to provide an optical power stabilizing feedback to the laser diode source.
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