WO1997035685A1

WO1997035685A1 - Laser light window cleaning

Info

Publication number: WO1997035685A1
Application number: PCT/US1997/005007
Authority: WO
Inventors: James W. Early
Original assignee: The Regents Of The University Of California
Priority date: 1996-03-28
Filing date: 1997-03-27
Publication date: 1997-10-02
Also published as: AU2592297A

Abstract

A laser light window cleaning method includes irradiating a laser light window (16) with a series of short duration, high peak power laser light pulses (12). The cleaning method may also include heating the window to at least 350 degrees Celsius prior to irradiation.

Description

LASER LIGHT WINDOW CLEANING

TECHNICAL FIELD

This invention relates to laser window cleaning.

This invention was made with government support under Contract No. W-7405-

ENG-36 awarded by the U.S. Department of Energy. The government has certain

rights in the invention.

BACKGROUND ART There have been developed various methods and apparatuses for laser ignition

of jet fuels, most of which require the ability to introduce laser light into the hostile

environment of jet engine combustion chambers. Laser light windows of silica glass

or quartz are located in combustor walls to isolate the laser and focusing optics from

the engine environment. The accumulation of fuel residues, in the form of soot and

unburned hydrocarbons, upon, a laser light window used to isolate the laser ignitor

from the hostile environment is inevitable. Direct contact with burning fuel aerosol

droplets results in hydrocarbon deposits on laser light windows. Another particularly

troublesome problem is caused by residue of the low volatility paraffinic components

of jet engine fuel which form high molecular weight, low volatility shellac-like

varnish on laser light windows during cooling down of a jet engine.

Therefore there is a need for an effective window cleaning method which can

be used to ensure effective transmission of laser light through the laser light window into the jet engine combustion chamber and which can be used to remove varnish from the laser light window prior to and during jet fuel ignition. It is an object of this invention to provide an effective window cleaning

method which can be used to ensure effective transmission of laser light through laser

light windows into jet engine combustion chambers.

It is another object of this invention to provide a method which can be used to

remove varnish from laser light windows prior to and during ignition.

Additional objects, advantages and novel features of the invention will be set

forth in part in the description which follows, and in part will become apparent to

those skilled in the art upon examination of the following or may be learned by

practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed

out in the appended claims.

DISCLOSURE OF INVENTION

To achieve the foregoing and other objects, and in accordance with the

purposes of the present invention, as embodied and broadly described herein, there

has been invented a laser light window cleaning method comprising contacting the

laser light window with a series of short duration, high peak power laser light pulses.

When needed, this laser light window cleaning method further comprises a first step in which the laser light window is heated to at least approximately 350°C. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying Figures, which are incorporated in and form a part of the

specification, illustrate some of the embodiments of the present invention and,

together with the description, serve to explain the principles of the invention. In the

drawings:

Figure 1 is a schematic representation of an embodiment of the apparatus used for laser light window cleaning.

Figure 2 is a photograph of (left to right): a window coated with carbonized

varnish with cleaned area , an uncarbonized varnish coated window, a sooted laser light window with a cleaned area.

Figure 3 is a graph of the percent transmission of laser light through a window

cleaned in accordance with the method of this invention.

BEST MODES FOR CARRYING OUT THE INVENTION

An effective laser ablation-based window cleaning method which utilizes the

same laser light used for ignition has been discovered. The same ablative property of the laser light that promotes fuel ignition also readily removes fuel combustion residues that collect on the laser window.

Jet fuels contain low volatility paraffinic components, typically as much as five percent. These low volatility components, which are in solution in the other jet

fuel components, are oxidized and carbonized during combustion with the carbonized components forming as sooty residue on the laser window glass. The sooty residue on the laser window glass can be removed by application of pulses of short duration high

peak power laser light such as that produced by a short-pulse Q-switched laser.

Short pulse lasers are defined in the art as being lasers designed to generate a

pulse of light lasting on the order of nanoseconds or less, and having very high power,

such as by Q switching or mode locking. Thus, the short duration, high peak power

laser light pulses of this application generally have the maximum instantaneous power

and last for nanoseconds or less. The high peak power density useful in this invention

can be from about 100kW/cm² to about 100 GW/cra². For best practice of the

invention, a peak power density range of about 10 MW/cnr to about 10 GW/cnr

presently preferred.

However, during cooldown periods when the jet engine is turned off, the low

volatility paraffinic components of the fuel come out of solution and form high

molecular weight, low volatility shellac-like varnish on laser light windows. This form of residue has the potential to collect upon window surfaces during a 20-minute

engine cool-down period in which temperatures within a turbojet combustor are

sufficiently high for migration of this material to the cooler surfaces of the laser light

window. Very thin varnishes are highly transmissive to laser light and would not pose a problem at engine start-up. Heavy coatings of this amber-brown material readily

transmit near infrared light but, depending upon coating thickness, the coating can

distort the laser mode sufficiently to degrade the laser induced ignition performance. Since the laser light window would normally be exposed to high temperatures while the engine is operating, varnishes are carbonized to sooty hydrocarbons rather

than forming up as shellac-like varnishes on the window during engine operation. Varnishes which have formed on the window during cooldown from earlier

combustion can be removed by heating of the window followed by laser light

treatment.

Heating of the window can be accomplished by utilization of heat from the

engine or, if made necessary by the rate and extent of deposition, by utilization of an

external heating coil positioned so as to uniformly heat the window.

Applying heat at temperatures of as much as approximately 350 °C carbonizes

the varnish residues, by hydrocarbon decomposition, generally in less than one

minute. After the varnish residues have been carbonized, the resulting sooty

hydrocarbon residues are removed with pulses of short duration high peak power laser

light. The sooty hydrocarbons readily absorb the laser light and vaporize from the

window surface.

This invention can be practiced using any source of laser light positioned so as

to contact the laser light window with a laser light beam, generally with the focal

plane of the laser light beam being a few centimeters beyond the laser light window

from the approach of the laser light beam. Presently most preferred are Q-switched

Nd:YAG lasers.

The following examples will demonstrate the operability of the invention. EXAMPLE I

In an evaluation of window cleaning in accordance with the method of this

invention, laser light windows were exposed to burning fuel in which a heavy coating

of fuel soaked soot and hydrocarbon residues were quickly deposited.

An apparatus set up as shown in Figure 1 was used to evaluate the ablative

cleaning characteristics of a short-duration high peak power laser light pulse of the

window contamination.

A Q-switched Nd:YAG laser light K) with a peak power of about 8 x IO⁶ W

was employed. Quartz substrates ( 1 in diameter x 1 mm thickness) were utilized as

laser light windows. Each window was immersed in the flame of burning Jet A fuel

for a period of 1/2 hour. The unprotected window faces were rapidly coated with a

layer of sooty residue. The actual thickness of this coating was not measured but

appeared to be at least 10 microns thick. A photograph of a coated window is shown

in Figure 2, the rightmost window in the photograph.

A contaminated window 14 was placed into the focused laser output 12 of the

short-duration high peak power laser light pulse at a series of selected distances from

the 10-cm focal length lens which was an integral part of a laser-based fuel ignitor.

Spot size diameters of the laser light at 0, 4 and 10 cm from the lens were 4.4 mm, 2.6

mm and 70 microns, respectively. The Nd:YAG laser provided a multiple-transverse

spatial mode with an output energy of 100 mJ within a 12 ns (FWHM) long, Q-

switched laser pulse.

The transmission of laser light 12 through the window 14 was monitored by a

laser pulse energy monitor 20 with each firing of the laser ignitor system. The laser pulse energy monitor 20 transmitted signals to an electronic readout device 22.

Transmission of 1.064 micron light through the window 14 with each laser shot was

determined by comparison of transmitted pulse energy to pulse energy transmitted by

a clean window.

The results of three window cleaning tests are shown in Figure 3, where

window transmission is plotted versus number of incident laser pulses for placement

of the window within the laser light at each of the various distances from the focusing

lens.

Although the highest rate of window residue removal was obtained when the

window was placed within the lens focal plane, the high power density of the ablative

light also occasionally caused optical damage to the window surface. This did not

occur with a clean window and was assumed to be due to or assisted by thermal

gradients established within the absorptive coating at the window surface.

It was found that at 4 cm distances from the lens the power density is low

enough to prevent window damage while maintaining adequate coating material

removal. As seen in Figure 3, the window transmission reaches a maximum of 95%

transmission within 20 laser pulses. We determined that the missing 5% transmission

was due to clipping of the laser's spatial mode in the low intensity wings where the

local light peak power is insufficient to remove residues. Substitution of an iris set to

the same aperture diameter as the ablative hole through the residue coating for the

coated window provided the same transmission.

A clean window was inserted after the lens in this measurement to compensate

for Fresnel losses at window surfaces. This result showed that no coating material remained in the cleaned area of the window which would otherwise absorb or scatter

laser light. A typical cleaned area of coated window is seen in the rightmost window of the photograph in Figure 2.

Examination of the laser cleaned area showed no visible contamination

remaining following the cleaning procedure and no optical damage was incurred by

the window surface. The small transmission difference between an uncontaminated window and the laser cleaned window was found to be due to vignetting of laser light

in the wings of the laser spatial mode in which local light intensity was insufficient to

ablate contaminants.

EXAMPLE II

A second series of tests were made to evaluate the performance of high peak

power laser light to remove window varnishes. Fuel varnish was prepared by

fractional distillation of Jet A fuel. The fuel varnish was contained in the low

volatility fraction of the fuel with a boiling point exceeding 300 °C. The distillation

product was an opaque, viscous, amber-colored liquid. The resulting material was

painted on a clean window in an about 0.5 mm thick layer which, upon cooling,

solidified to a shellac-like varnish as shown in the center window in the photograph of Figure 2. The transmission of this varnish was determined to be 65% and could not

be readily ablated by use of the Q-switched Nd.YAG laser light even at the incident

power density existing at the lens focal plane.

It was found that this coating material could readily be carbonized by a short

exposure to temperatures of 350 °C (temperatures at the wall of a turbojet combustor where this window would be located typically exceed this value). After several

minutes the varnish had decomposed to an optically dense, black carbon-containing

material which was highly absorptive of Nd:YAG light, as shown in the leftmost

window of the photographs of Figure 2.

After the shellac-like varnish was decomposed into the black carbonized

material, laser light was applied in a series of pulses as described in Example I.

Nd: YAG laser light window cleaning performance virtually identical to that

obtained in Example I was achieved.

While the apparatuses, articles of manufacture, methods and compositions of

this invention have been described in detail for the purpose of illustration, the

inventive apparatuses, articles of manufacture, methods and compositions are not to

be construed as limited thereby. This patent is intended to cover all changes and

modifications within the spirit and scope thereof.

INDUSTRIAL APPLICABILITY

The methods and apparatuses of this invention can be employed for cleaning

of laser light windows used in walls of laser ignited fuel combustion chambers.

Claims

WHAT IS CLAIMED IS:

1. A method for cleaning a laser light window comprising;

contacting said laser light window with at least one short duration, high

peak power laser light pulse, thereby removing any residue from said window.

2. A method as recited in Claim 1, wherein said laser light window is

contacted with a series of short duration, high peak power laser light pulses.

3. A method for cleaning a laser light window comprising:

(a) heating said laser light window; thereafter

(b) contacting said laser light window with at least one short

duration, high peak power laser light pulse,

thereby removing any varnish from said window.

4. A method as recited in Claim 3 wherein said laser light window is

heated to approximately 350° in step (a).

5. A method as recited in Claim 3, wherein said laser light window is

contacted with a series of short duration, high peak power light pulses.