Background

When building a laser, it is important to minimize cavity loss from every optical component inside the laser cavity. Anti-reflection (AR) coatings are often used to reduce the reflection losses from optical surfaces. AR coatings are sometimes, however, not ideal. For example, many UV lasers are made using intracavity harmonic generation. AR coatings that are exposed to UV beams will degrade over the time and cause issues with respect to the service lifetime of the laser.

Some laser applications require a single longitudinal mode (SLM) laser, which is often made possible by adding a mode selection element, such as an etalon, a Lyot filter etc.

Introducing an extra element to the laser cavity, however, increases the cost and complexity as well as causing additional cavity loss.

Summary of the invention

The present invention provides a way to use uncoated optical components inside a laser cavity to avoid AR coatings. It can also be used to achieve SLM with no additional component.

Description of the drawing

The invention will be described with respect to a drawing in several figures, in which to the extent possible, like reference numerals are used across the figures.

• Figure 1 shows an example of an intracavity second harmonic generation (SHG) laser of the prior art.

• Figure 2 shows the transmission curve of an uncoated etalon.

• Figure 3 shows the same laser cavity as in Figure 1 except that items 2 and 3 are made of uncoated etalons 32 and 33, respectively.

• Figure 4 shows a separation of combined transmission peaks S being wide enough that the adjacent peak is at least essentially out of the laser gain bandwidth.

• Figure 5 shows the use of an etalon 51 to replace item 1 in Figure 3. • Figure 6 shows an example of transmission of such an etalon 51.

• Figure 7 shows the elimination of the remaining coated optic in Figure 3, namely item 4, if the fundamental beam and the harmonic beam are cross polarized.

Detailed Description

Figure 1 shows an example of an intracavity second harmonic generation (SHG) laser of the prior art. Item 1 is a high reflector for both the fundamental beam and the second harmonic beam. Item 4 is the output coupler, which reflects the fundamental beam and transmits the second harmonic beam. Item 2 is the laser gain medium for the fundamental beam. Item 3 is the SHG optic. Item 5 is the circulating fundamental beam. Item 6 is the second harmonic beam. Surfaces of items 2 and 3 are AR coated to reduce reflection loss.

If the harmonic beam is in the UV range, at least some optical components are exposed to the UV beam in an intracavity harmonic generation laser. In the case of Figure 1, all optical components are exposed to the UV beam if the second harmonic beam is in the UV range. Coatings on these optics will degrade over the time and shorten the lifetime of the laser. If on the other hand they are not coated, each uncoated surface causes significant reflection loss of ~4% depending on the material refractive index, incidence angle, and polarization.

The present invention provides a way to avoid AR coatings for the fundamental wavelength while avoiding most of the significant reflection loss just mentioned. The idea is based on the uncoated etalon. Figure 2 shows the transmission curve of an uncoated etalon. FSR is the free spectral range. The transmission is 100% for a perfect etalon at its transmission peak. If an optical component inside the laser cavity is made to an etalon, cavity longitudinal modes that are close the transmission peak would experience little reflection loss. Hence, no AR coating is necessary. SLM operation can also be achieved by choosing an appropriate free spectral range (FSR) to select only one longitudinal mode and suppress other longitudinal modes.

Figure 3 shows the same laser cavity as in Figure 1 except that items 2 and 3 are made of uncoated etalons 32 and 33, respectively. The pump source and scheme for item 32 is omitted for clarity in Figure 3 because the present invention applies to all pump sources and schemes. Only those cavity longitudinal modes that are close to the transmission peaks of both items 32 and 33 may lase. This property can also be used to obtain SLM operation. The FSR’s of items 32 and 33, and the FSR difference between them can be chosen so that, as illustrated in Figure 4, the separation of the combined transmission peaks S is wide enough that the adjacent peak is at least essentially out of the laser gain bandwidth. In the meantime, the central peak is narrow enough that there is significant transmission difference between adjacent cavity longitudinal modes so that only the cavity longitudinal mode close to the transmission peak can lase. Hence the laser can operate in SLM.

The alert reader appreciates that item 1 in Figure 3 is still coated and thus continues to be exposed to the UV beam. The present invention also provides a solution to that problem, as illustrated in Figure 5. Item 51 is an etalon that can be used to replace item 1 in Figure 3. The high reflection (HR) coating for both the fundamental and second harmonic beams (items 5 and 6 in Figure 5) is on one side of item 51 while the other side is uncoated. The UV beam incidents on (strikes) the coating from the optical material side instead of the air side. This will greatly slow the coating degradation because the main cause of the degradation is the interaction between the UV beam and the contamination deposited onto the coating from the air. An example of transmission of such an etalon is shown in Figure 6, which is calculated using 99.9% reflectivity for the HR coating and 3.4% reflectivity for the uncoated surface. It is a good high reflector.

There is one more coated optic, item 4, in Figure 3. The present invention also provides a solution to that problem if the fundamental beam and the harmonic beam are cross polarized, as illustrated in Figure 7. Item 74 in the invention is intended to replace item 4 in Figure 3. The fundamental laser beam is set at p polarization. Item 74 is made of birefringent material. Its surface facing the inside of the cavity is uncoated and set at the Brewster angle for the fundamental beam. The other surface of item 74 is coated as HR (high reflection) for the fundamental beam and AR (anti reflection) for the harmonic beam. The optical axis of the material is oriented in such a way that one of the two beams (fundamental and harmonic beams) is ordinary light while the other is extraordinary light. The birefringence separates the harmonic beam from the fundamental beam inside item 74. The length L is chosen so that the two beams hit different spots on the coating. The coating degradation at the spot where the UV beam hits has no effect on the fundamental laser cavity. Even a shorter length L would help because the two beams are only partially overlapped. The present invention is especially useful if the harmonics are in the UV range. For example, Pr:YLF has many lasing wavelengths in the visible range. Second harmonics of these lasing wavelengths are in the range from UVA to UVC. Applying this invention would help to avoid degradation or damage to the coating.

The alert reader, informed by the teachings herein, will have no difficulty devising myriad obvious variants and improvements, all of which are intended to be encompassed within the scope of the claims which follow.